GaInP epitaxial stacking structure and fabrication method thereof and a FET transistor using this structure

ABSTRACT

A GaInP epitaxial stacking structure and fabrication method thereof, and a FET transistor using this structure are provided wherein, stacked upon a GaAs single-crystal substrate are at least a buffer layer, a Ga Z In 1−Z As (0&lt;Z≦1) channel layer, and a Ga Y In 1−Y P (0&lt;Y≦1) electron-supply layer joined to the channel layer, wherein the GaInP epitaxial stacking structure includes a region within the electron-supply layer wherein the gallium composition ratio (Y) decreases from the side of the junction interface with the channel layer toward the opposite side.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is an application filed under 35 U.S.C. §111 (a)claiming benefit pursuant to 35 U.S.C. §119 (e) (1) of the filing datesof the Provisional Application Nos. 60/159,652, 60/163,285, 60/166,758,and 60/174,824 filed Oct. 18, 1999, Nov. 3, 1999, Nov. 22, 1999 and Jan.7, 2000 pursuant to 35 U.S.C. §111 (b) respectively.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a GaInP epitaxial stacking structure,and more specifically to a GaInP epitaxial stacking structure for FETsand a fabrication method thereof, which has electron-supply layers andspacer layers which give high-mobility characteristics, and ahigh-mobility field effect transistor using this structure.

[0004] 2. Description of the Prior Art

[0005] Schottky junction-type field effect transistors (known asMESFETs) which operate in the microwave region or millimeter wave regioninclude GaInP high electron mobility transistors (known as TEGFETs,MODFETs and the like) which utilize mixed crystals of gallium-indiumphosphide (Ga_(A)In_(1−A)P: 0≦A≦1) (see IEEE Trans. Electron Devices,Vol. 37, No. 10 (1990), pp. 2141-2147). GaInP MODFETs can be used aslow-noise MESFETs for signal amplification in the microwave region (seeIEEE Trans. Electron Devices, Vol. 46, No. 1 (1999), pp. 48-54) and aspower MESFETs for transmission applications (see IEEE Trans. ElectronDevices, Vol. 44, No. 9 (1997), pp. 1341-1348).

[0006]FIG. 1 is a schematic diagram of the cross-sectional structure ofa conventional GaInP TEGFET. The substrate 10 used is made ofsemi-insulating gallium arsenide (chemical formula: GaAs) with a {001}crystal plane as its primary plane. Upon the substrate 10 is deposited abuffer layer 11 consisting of a high-resistance Group III-V compoundsemiconductor layer. Upon the buffer layer 11 is deposited an electrontransporting layer (channel layer) 12 consisting of n-type mixedcrystals of gallium-indium arsenide (Ga_(Z)In_(1−Z)As: 0<Z≦1). A spacerlayer may be deposited upon the channel layer 12, but particularly inpower TEGFETs for transmission applications, an electron-supply layer 13consisting of mixed crystals of gallium-indium phosphide(Ga_(Y)In_(1−Y)P: 0<Y≦1) is deposited without an interposed spacerlayer. The carrier (electron) density of the electron-supply layer 13 isadjusted by the intentional addition (doping) of silicon (Si) or othern-type impurities which are not readily diffused. Upon theelectron-supply layer 13, a contact layer 14 consisting of n-type GaAsor the like is typically provided in order to form the low-contactresistance source electrode 15 and drain electrode 16. In addition,between the source and drain electrodes 15, 16, the contact layer 14 ispartially removed to expose a recess structure, and a Schottkyjunction-type gate electrode 17 is provided, thereby constituting aTEGFET.

[0007] The various constituent layers 11-14 which constitute the GaInPepitaxial stacking structure 1A for MODFET application illustrated inFIG. 1, because of their ease of film formation, are conventionallyformed by the metal-organic chemical vapor deposition (MOCVD) method(see ibid IEEE Trans. Electron Devices, Vol. 44 (1997)). Among theseconstituent layers, the electron-supply layer 13 is a functional layerfor supplying electrons formed to accumulate as a two-dimensionalelectron gas (TEG) in the vicinity of the junction interface 12 a of thechannel layer 12. The electron-supply layer 13 is conventionally formedof gallium-indium phosphide (Ga_(Y)In_(1−Y)P: 0<Y≦1) doped with silicon(the symbol of element: Si) or other n-type impurities which are notreadily diffused (see ibid IEEE Trans. Electron Devices, Vol. 44(1997)). The carrier density (units: cm⁻³) of the electron-supply layer13 is commonly made 1-3×10¹⁸ cm⁻³ or 2×10¹⁸ cm⁻³ in particular. Thethickness of the layer is typically set within the range 10 nm to 40 nm.In addition, in a GaInP TEGFET, the n-type electron-supply layer isnormally constituted from Ga_(Y)In_(1−Y)P (0<Y≦-1) layers wherein thegallium composition ratio (=Y) is fixed in the layer thicknessdirection.

[0008] In addition, in the structure wherein a spacer layer is depositedupon the channel layer 12, in order to prevent the two-dimensionalelectron gas from being disturbed due to ionization scattering from thechannel layer 12, the spacer layer is a functional layer provided forthe spatial isolation of the channel layer 12 and electron-supply layer13 (see “Physics and Applications of Semiconductor Superlattices,”Physical Society of Japan, ed. (published by Baifukan, Sep. 30, 1986,first edition, fourth printing), pp. 236-240). In a GaInP TEGFET, thespacer layer is typically constituted from undoped Ga_(X)In_(1−X)P(0<X≦1) (see ibid IEEE Trans. Electron Devices, Vol. 44 (1997)).Regardless of the case of GaInP TEGFET, spacer layers are constitutedfrom high-purity undoped layers with a low total amount of impurities,and their layer thickness is typically in the range from 2 nanometers(nm) to 10 nm (see ibid “Physics and Applications of SemiconductorSuperlattices,” pp. 18-20).

[0009] For example, in a low-noise GaInP TEGFET, the noise-figure (NF)and other major properties vary depending on the electron mobility, sothe higher the electron mobility, the lower the NF conveniently becomes.For this reason, in order to cause the electrons supplied from then-type electron-supply layer 13 to accumulate as a two-dimensionalelectron gas in the interior regions of the Ga_(Z)In_(1−Z)As (0<Z≦1) inthe vicinity of the junction interface with the spacer layer consistingof undoped Ga_(X)In_(1−X)P (0<X≦1), the composition at the junctioninterface between the channel layer 12 and the spacer layer must changeabruptly and exhibit high electron mobility.

[0010] In addition, the formation of a buffer layer is typicallyperformed by vapor deposition without varying the starting materialspecies of gallium (element symbol: Ga). Since the admixture of carbon(element symbol: C) acceptors that electrically compensate residualdonor components represented by silicon occurs readily, and ahigh-resistance GaAs layer or Al_(L)Ga_(1−L)As layer is easily obtainedin the undoped state (see J. Crystal Growth, 55 (1981), pp. 255-262),trimethyl gallium (chemical formula: (CH₃)₃Ga) is used as the gallium(Ga) source (see J. Crystal Growth, 55 (1981), pp. 246-254, ibd, pp.255-262, and PCT application publication No. 10-504685).

[0011] In a GaInP TEGFET for low-noise amplification, the noise-figure(NF) and other major properties vary depending on the two-dimensionalelectron mobility (units: cm²/V·s), so the higher the electron mobility(cm²/V·s), the lower the NF becomes. For this reason, in a low-noiseTEGFET, the electron-supply layer which takes the role of supplyingelectrons must be constituted from Ga_(Y)In_(1−Y)P (0<Y≦1) which canexhibit a high electron mobility. On the other hand, in a power TEGFET,from the standpoint of causing it to operate with a relatively largesource-drain current flowing, a large sheet carrier density (units:cm⁻²) is required together with the electron mobility. Therefore,electron-supply layer for power TEGFET applications must be constitutedfrom a Ga_(Y)In_(1−Y)P (0<Y≦1) layer that exhibits a high sheet carrierdensity.

[0012] However, in the conventional electron-supply layer consisting ofGa_(Y)In_(1−Y)P wherein the gallium composition ratio (=Y) or indiumcomposition ratio (=1−Y) is roughly constant, at a relatively high sheetcarrier density, there is a disadvantage in that a high electronmobility cannot be manifested stably. For this reason, in low-noiseGaInP TEGFETs for example, a large transconductance (g_(m)) is notobtained, thus obstructing the stable supply of low-noise GaInP TEGFETswith a superior low noise-figure (NF).

[0013] A first object of the present invention is to provide a GaInPepitaxial stacking structure containing a Ga_(Y)In_(1−Y)P (0<Y≦1)electron-supply layer and fabrication method thereof for stablymanifesting a high electron mobility in excess of 5000 cm²/V·s at roomtemperature and at a relatively high sheet carrier density of 1.5×10¹²cm⁻² or greater and 2.0×10¹² cm⁻² or less. With this structure,low-noise GaInP high electron mobility transistors with superiortransconductance properties and power GaInP TEGFETs with superior powertransformation efficiency due to their high source-drain current can beprovided.

[0014] In addition, in a structure wherein a spacer layer is providedbetween the channel layer and electron-supply layer, if aGa_(X)In_(1−X)P spacer layer wherein the indium composition ratio (=1−X)is roughly constant is provided joined to the Ga_(Z)In_(1−Z)As (0<Z≦1)channel layer 12, mutual diffusion occurs between phosphorus (elementsymbol: P) and arsenic (element symbol: As) in the vicinity of thejunction interface 12 a, so a problem occurs in that the steep change incomposition at the junction interface 12 a is worsened.

[0015] If the steepness of change in composition at the junctioninterface 12 a is not achieved, a two-dimensional electron gas does notefficiently accumulate in the interior regions of the Ga_(Z)In_(1−Z)Aschannel layer 12, and the electron mobility drops. The electron mobilityparticularly affects the transconductance (g_(m)) of GaInP TEGFETs forlow-noise amplification, and influences the noise-figure (NF) even more.At a low electron mobility, a high g_(m) is not obtained and therefore,a GaInP TEGFET with a low NF is not obtained.

[0016] In addition, it was conventionally common for the spacer layer tobe constituted from an undoped Ga_(X)In_(1−X)P (0<X≦1) layer wherein theindium composition ratio is constant. However, the carrier density inthe undoped state is roughly 1×10¹⁶ cm⁻³ at the lowest. Since thetwo-dimensional electron gas accumulates more efficiently by loweringthe carrier density of the spacer layer, in order for a high electronmobility to be manifested, the spacer layer must be constituted from aGa_(X)In_(1−X)P (0<X≦1) layer with an even lower carrier density.

[0017] Thus, a second object of the present invention is to provide anepitaxial stacking structure comprising a spacer layer made ofGa_(X)In_(1−X)P (0<X≦1) which can stably manifest an even higherelectron mobility and has a low carrier density. With this structure, itis possible to provide a GaInP epitaxial stacking structure withexcellent transconductance.

[0018] Irrespective of GaInP TEGFETs, the transconductance (g_(m)) andpinch-off characteristics of high electron mobility field effecttransistors are known to fluctuate depending on the quality of thebuffer layer. For example, in the normal AlGaAs/GaAs lattice-matchedTEGFETs and AlGaAs/GaInAs strained-lattice TEGFETs, a high g_(m) andgood pinch-off characteristics are obtained, and the buffer layer isformed as a high-resistance layer with a low leakage current.

[0019] On the other hand, as described above, in a GaInP TEGFETcomprising an electron-supply layer consisting of Ga_(Y)In_(1−Y)P whichis one type of a phosphorus- (element symbol: P) containing Group III-Vcompound semiconductor, simply making the buffer layer a high-resistancelayer has conventionally had the problem wherein a homogenous g_(m) andpinch-off voltage cannot be stably obtained. The present inventorsdiscovered that this instability of properties derives fromheterogeneity in the indium composition ratio (=1−Y) of theGa_(Y)In_(1−Y)P electron-supply layer due to differences in the gallium(Ga) source utilized in the formation of the buffer layer of asuperlattice structure that uses AlGaAs and GaAs in particular asconstituent layers.

[0020] In addition, in the buffer layers consisting of the conventionalconstitution such as AlGaAs/GaAs superlattice-structure buffer layers,there are problems regarding the DC properties (static properties) ofthe transistor in that fluctuation in the source-drain current valueunder illumination (so-called “photoresponsibility”) (see G. J. Ree,ed., Semi-Insulating III-V Materials, (Shiva Pub. Ltd. (Kent, UK, 1980),pp. 349-352) and “hysteresis” of the source-drain current (see MakotoKikuchi, Yasuhiro Tarui, eds., “Illustrated Semiconductor Dictionary,”(Nikkan Kogyo Shimbunsha, Jan. 25, 1978), p. 238) and “kinks” easilyoccur (JP-A-10-247727 and JP-A-10-335350).

[0021] Therefore, a third object of the present invention is to providean epitaxial stacking structure comprising a buffer layer for forming aGa_(Y)In_(1−Y)P (0<Y≦1) electron-supply layer that has high resistancesuitable for reducing the leakage current and that has a homogeneousindium composition.

[0022] In a GaInP TEGFET, the spacer layer is constituted fromGa_(X)In_(1−X)P (0<X≦1) which is an indium-containing Group III-Vcompound semiconductor, and moreover it is constituted as a thin film.The conventional MOCVD technology has a problem in that thin-film spacerlayers with a homogenous indium composition ratio (=1−X) cannot bestably obtained.

[0023] For this reason, conventional GaInP high electron mobility fieldeffect transistors which use as the spacer layer a Ga_(X)In_(1−X)P(0<X≦1) layer wherein the indium composition ratio is not sufficientlyhomogenous cannot maintain a homogenous band offset with the channellayer due to “fluctuation” in the indium composition ratio within thespacer layer, and for this reason, achieving a homogenoustransconductance (g_(m)) and pinch-off voltage was difficult.

[0024] Therefore, a fourth object of the present invention is to providean epitaxial stacking structure for TEGFET applications that has aGa_(X)In_(1−X)P (0<X≦1) spacer layer with a superior homogeneity in itsindium composition. With this structure, it is possible to provide aGaInP high electron mobility transistor with superior homogeneity in itspinch-off voltage and other properties.

SUMMARY OF THE INVENTION

[0025] In order to achieve these objects, the present invention providesa GaInP epitaxial structure stacked upon a GaAs single-crystalsubstrate, comprising at least a buffer layer, a Ga_(Z)In_(1−Z)As(0<Z≦1) channel layer, and a Ga_(Y)In_(1−Y)P (0<Y≦1) electron-supplylayer provided joined to the channel layer, the GaInP epitaxial stackingstructure including a region within the electron-supply layer whereinthe gallium composition ratio (Y) decreases from the side of thejunction interface with the channel layer toward the opposite side.

[0026] The gallium composition ratio of the aforementionedelectron-supply layer is Y≧0.51±0.01.

[0027] In addition, the gallium composition ratio of the aforementionedelectron-supply layer at the junction interface with the channel layeris Y≧0.70.

[0028] Moreover, the gallium composition ratio of the aforementionedelectron-supply layer at the junction interface with the channel layeris Y=1.0.

[0029] Furthermore, at the junction interface between the aforementionedelectron-supply layer and the channel layer, there is a region with athickness in the range 1-20 nanometers wherein the gallium compositionratio is constant.

[0030] In accordance with another aspect, the invention provides a GaInPepitaxial structure upon a GaAs single-crystal substrate, comprising atleast a buffer layer, a Ga_(Z)In_(1−Z)As (0<Z≦1) channel layer, aGa_(X)In_(1−X)P (0<X≦1) spacer layer, and a Ga_(Y)In_(1−Y)P (0<Y≦1)electron-supply layer, wherein the channel layer, spacer layer, andelectron-supply layer join each other in this order, and the GaInPepitaxial stacking structure includes a region within the spacer layerwherein the gallium composition ratio (X) decreases from the side of thejunction interface with the channel layer toward the side of theelectron-supply layer.

[0031] The gallium composition ratio of the aforementionedelectron-supply layer is Y=0.51±0.01.

[0032] In addition, the gallium composition ratio of the aforementionedspacer layer at the junction interface with the channel layer is X≧0.70.

[0033] Moreover, the gallium composition ratio of the aforementionedspacer layer at the junction interface with the channel layer is X=1.0.

[0034] Furthermore, the gallium composition ratio of the aforementionedspacer layer at the junction interface with the channel layer isX=0.51±0.01.

[0035] In addition, a boron-doped n-type layer constitutes theaforementioned spacer layer.

[0036] Furthermore, the aforementioned buffer layer consists of aperiodic structure of a plurality of Al_(L)Ga_(1−L)As (0≦L≦1) layerswith different aluminum composition ratios (L) vapor-deposited using anorganic methyl compound of aluminum or gallium as its starting material,having an Al_(M)Ga_(1−M)As (0≦M≦1) layer vapor-deposited or the periodicstructure using an organic ethyl compound of aluminum or gallium as itsstarting material.

[0037] In addition, the relationship 0.9≦K≦1.0 holds true for thecompensation ratios (K) (K=N_(a)/N_(d) (if N_(a)≦N_(d)) andK=N_(d)/N_(a) (if N_(d)<N_(a)); N_(a): acceptor density of theconstituent layer, N_(d): donor density of the constituent layer) of theconstituent layers of the periodic structure.

[0038] The aforementioned periodic structure consists of anAl_(L)Ga_(1−L)As (0≦L≦1) layer and a p-type GaAs layer, and the carrierdensity of each constituent layer is 1×10¹⁵ cm⁻³ or less.

[0039] In addition, the aforementioned Al_(M)Ga_(1−M)As layer istouching the channel layer.

[0040] Moreover, the aforementioned Al_(M)Ga_(1−M)As layer has a carrierdensity of 5×10¹⁵ cm⁻³ or less, thickness of 100 nm or less and consistsof an n-type layer.

[0041] Furthermore, the thickness of the aforementioned Al_(M)Ga_(1−M)Aslayer is less than the thickness of the constituent layers of theperiodic structure.

[0042] In addition, the aluminum composition ratio (M) of theaforementioned Al_(M)Ga_(1−M)As layer is less than the aluminumcomposition ratio (L) of the Al_(L)Ga_(1−L)As layers which constitutethe periodic structure.

[0043] Moreover, the aforementioned buffer layer comprises anAl_(L)Ga_(1−L)As (0≦L≦1) layer vapor-deposited using a trimethylcompound of a Group III element as its starting material, a GaAs layervapor-deposited using triethyl gallium as the starting material forgallium is disposed between the buffer layer and channel layer, thechannel layer has a conduction type of n-type, the spacer layer andelectron-supply layer are n-type layers vapor-deposited using trimethylgallium as the starting material for gallium, the homogeneity in theindium composition ratio within each of the spacer layer andelectron-supply layer is ±2% or less, and the spacer layer andelectron-supply layer are touching each other.

[0044] In addition, the surface roughness (haze) after formation of theaforementioned channel layer is 60 ppm or less, and the channel layertouches a GaAs layer vapor-deposited using triethyl gallium as thestarting material for gallium.

[0045] Furthermore, the aforementioned spacer layer and channel layertouch each other, and the surface roughness (haze) after formation ofthe spacer layer is 100 ppm or less.

[0046] In addition, the surface roughness (haze) after formation of theelectron-supply layer is 200 ppm or less.

[0047] In accordance with another aspect, the present invention providesa method of fabricating a GaInP epitaxial stacking structure comprising:a step wherein the buffer layer is vapor-deposited using an organicmethyl compound of aluminum or gallium as its starting material, a stepwherein the AlGaAs layer is vapor-deposited using an organic ethylcompound of aluminum or gallium as its starting material in contact withthe periodic structure, and a step wherein the channel layer andelectron-supply layer are formed by means of a chemical vapor depositionmethod using cyclopentadienyl indium which has a bond valence ofmonovalent as the starting material for indium.

[0048] In accordance with another embodiment, the present inventionprovides a method of fabricating a GaInP epitaxial stacking structurecomprising: a step wherein the buffer layer is vapor-deposited using anorganic methyl compound of aluminum or gallium as its starting material,a step wherein the Al_(M)Ga_(1−M)As (0≦M≦1) layer is vapor-depositedusing an organic ethyl compound of aluminum or gallium as its startingmaterial in contact with the periodic structure, and a step wherein thechannel layer, spacer layer and electron-supply layer are formed bymeans of a chemical vapor deposition method using cyclopentadienylindium which has a bond valence of monovalent as the starting materialfor indium.

[0049] Moreover, the present invention also comprises a field effecttransistor fabricated using the aforementioned GaInP epitaxial stackingstructure.

[0050] As described above, the present invention constitutes theelectron-supply layer as a Ga_(Y)In_(1−Y)P layer with a gradient in thecomposition such that the gallium composition ratio decreases in thedirection of increasing layer thickness from the channel layer towardthe contact layer, so a two-dimensional electron gas efficientlyaccumulates in the interior of the channel layer, and a high electronmobility is manifested, so a GaInP epitaxial stacking structure with asuperior homogeneity in the transconductance and pinch-off voltage canbe provided.

[0051] In addition, as described above, the present inventionconstitutes the spacer layer as a Ga_(X)In_(1−X)P layer with a gradientin the composition such that the gallium composition ratio decreases inthe direction of increasing layer thickness from the channel layertoward the contact layer, so a two-dimensional electron gas efficientlyaccumulates in the interior of the channel layer, and a high electronmobility is manifested, so a GaInP epitaxial stacking structure with asuperior homogeneity in the transconductance and pinch-off voltage canbe provided.

[0052] Moreover, as described above, the present invention constitutesthe superlattice periodic structure constituting one part of the bufferlayer with a periodic alternating layer structure of Al_(L)Ga_(1−L)Aslayers vapor-deposited using an organic methyl compound as its startingmaterial and with a stipulated compensation ratio, so a GaInP epitaxialstacking structure with a low leakage current can be provided.

[0053] Furthermore, the constitution is such that an indium-containingGroup III-V compound is provided with a GaAs thin-film layervapor-deposited from triethyl gallium as its starting material, so aGa_(Z)In_(1−Z)As channel layer, a Ga_(X)In_(1−X)P spacer layer, and anelectron-supply layer with superior homogeneity in indium compositioncan be formed, and therefore, a GaInP epitaxial stacking structure witha superior homogeneity in the transconductance and pinch-off voltage canbe provided.

[0054] The above and other objects and features of the invention willbecome apparent from the following description made with reference tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055]FIG. 1 is a schematic cross section of the structure for aconventional GaInP TEGFET.

[0056]FIG. 2 is a chart illustrating the pattern of the gradient ingallium composition for the Ga_(Y)In_(1−Y)P gradient-compositionelectron-supply layer.

[0057]FIG. 3 is a schematic cross section of a GaInP TEGFET used toexplain a preferred embodiment of the present invention.

[0058]FIG. 4 is a chart illustrating the pattern of the gradient ingallium composition for the Ga_(X)In_(1−X)P gradient-composition spacerlayer.

[0059]FIG. 5 is a schematic cross section of a GaInP TEGFET used toexplain a preferred embodiment of the present invention.

[0060]FIG. 6 is a schematic cross section of a GaInP epitaxial stackingstructure used to explain a preferred embodiment of the presentinvention.

[0061]FIG. 7 is a schematic cross section of a GaInP TEGFET used toexplain a preferred embodiment of the present invention.

[0062]FIG. 8 is a schematic cross section of the epitaxial structure fora GaInP TEGFET used to explain a preferred embodiment of the presentinvention.

[0063]FIG. 9 is a schematic cross section of a GaInP TEGFET recited in aworking example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] The basic constitution of the epitaxial stacking structure forGaInP FETs according to the present invention has a structure stackedupon the surface of a GaAs single crystal 301, comprising at least abuffer layer 302, a Ga_(Z)In_(1−Z)As (0<Z≦1) channel layer 303 and aGa_(Y)In_(1−Y)P (0<Y≦1) electron-supply layer 304 provided joined tothis channel layer (FIG. 3) or has a structure stacked upon the surfaceof a GaAs single crystal 601, comprising at least a buffer layer 602, aGa_(Z)In_(1−Z)As (0<Z≦1) channel layer 603, a Ga_(X)In_(1−X)P (0<X≦1)spacer layer 604 provided joined to this channel layer and aGa_(Y)In_(1−Y)P (0<Y≦1) electron-supply layer 605 provided joined tothis spacer layer (FIG. 5).

[0065] It is preferable that a semi-insulating {001} substrate inparticular be used as the GaAs single-crystal substrate.

[0066] In the first preferred embodiment according to claim 1 of thepresent invention, the Ga_(Y)In_(1−Y)P (0<Y≦1) electron-supply layer maybe formed by means of atmospheric-pressure or low-pressure MOCVD orother vapor-deposition method using trimethyl gallium (chemical formula:(CH₃)₃Ga) as the gallium (Ga) source, trimethyl indium (chemicalformula: (CH₃)₃In) as the indium (In) source and phosphine (chemicalformula: PH₃) as the phosphorus (P) source, for example. Triethylgallium (chemical formula: (C₂H,)₃Ga) can also be used as the gallium(Ga) source. The Ga_(Y)In_(1−Y)P layer may be formed using a(CH₃)₃Ga/C₅H₅In/PH₃ MOCVD reaction using cyclopentadienyl indium(chemical formula: C₅H₅In) (see JP-A-8-17160) as the indium (In) source,for example. The Ga_(Y)In_(1−Y)P electron-supply layer(gradient-composition layer) with a gradient in the gallium compositionsuch that the gallium composition ratio (Y) decreases in the directionof increasing layer thickness from the junction interface with theGa_(Z)In_(1−Z)As (0<Z≦1) channel layer can be formed by reducing theamount (concentration) of the gallium source provided to the MOCVDreaction system with increasing film deposition time while maintaining aconstant amount (concentration) of the indium source supplied to theMOCVD reaction system. In addition, film formation can be performed byincreasing the amount of the indium source supplied together withincreasing film deposition time while maintaining a constant amount ofthe gallium source supplied to the MOCVD reaction system. In addition,in order to obtain an electron-supply layer with the desired carrierdensity, it is preferable to perform doping with silicon (Si) or thelike during deposition.

[0067]FIG. 2 schematically illustrates the pattern of the gradient ingallium composition in the interior of the electron-supply layerconsisting of a Ga_(Y)In_(1−Y)P gradient-composition layer. The gradientpattern in the gallium composition illustrated in FIG. 2 is one exampleof the gradient-composition pattern allowable by the present invention,and in this figure, (a) shows the change in the gallium composition whenthe gallium composition is varied uniformly and linearly together withthe increase in the thickness of the electron-supply layer. The symbol(b) shows the gradient pattern in the case in which the galliumcomposition is kept constant in the vicinity of the junction interfacewith the channel layer and then the gallium composition is reduceduniformly and linearly. In addition, the symbol (c) shows an example inthe case in which the gallium composition is reduced in a curved manner.In addition, the symbol (d) is a gradient pattern in the case in whichthe gallium composition is reduced in a step-wise manner.

[0068] The gradient pattern is not limited to the patterns illustratedin FIG. 2, but in the third preferred embodiment according to claim 3 ofthe present invention, the gallium composition ratio (=Y) of theGa_(Y)In_(1−Y)P electron-supply layer at the junction interface with theGa_(Z)In_(1−Z)As (0<Z≦1) channel layer is 0.70 or greater or preferably0.85 or greater. This is because, by making the gallium compositionratio (=Y) 0.70 or greater, the mobility of the two-dimensional electrongas accumulated in the channel layer can be increased. If the gradientpattern for the gallium composition shown by symbol (d) in FIG. 2 isfollowed, for example, the gallium composition ratio (=Y) may be made0.90 in the region wherein the thickness from the junction interfacewith the channel layer is up to 2 nm, and next the gallium compositionratio can be made 0.70 in another 2-nm thick region, and then another2-nm thick region with a gallium composition of 0.51 can be used to givea multi-layer constitution of individual Ga_(Y)In_(1−Y)P layers, and bythus reducing the gallium composition layer step-wise by 0.2, anelectron-supply layer having a gradient composition according to thispreferred embodiment can be formed.

[0069] Particularly in the fourth preferred embodiment according toclaim 4 of the present invention, the gallium composition ratio (=Y) ofthe Ga_(Y)In_(1−Y)P electron-supply layer at the junction interface withthe Ga_(Z)In_(1−Z)As (0<Z≦1) channel layer is set to 1.0. Setting thegallium composition ratio to 1.0 namely makes the electron-supply layergallium phosphide (chemical formula: GaP). If the gradient pattern forthe gallium composition shown by symbol (b) in FIG. 2 is followed, forexample, the gallium composition ratio (=Y) may be set to 1.0 in theregion wherein the thickness from the junction interface with thechannel layer is up to 2 nm, and then the gallium composition ratio canbe reduced linearly to 0.51 so that an electron-supply layer having agradient composition according to this preferred embodiment can beformed. By setting the gallium composition ratio to 1.0 at the junctioninterface with the channel layer, a high junction barrier with theGa_(Z)In_(1−Z)As channel layer is formed so that a two-dimensionalelectron gas can be efficiently accumulated.

[0070] In the second preferred embodiment according to claim 2 of thepresent invention, in the gradient-composition layer consisting ofGa_(Y)In_(1−Y)P with a pattern wherein the gallium composition ratio(=Y) decreases with increasing layer thickness, the minimum galliumcomposition ratio (=Y) reached when decreasing is set to 0.51±0.01. Forexample, consider forming an electron-supply layer from an n-typeGa_(Y)In_(1−Y)P gradient-composition layer wherein the galliumcomposition ratio (=Y) is decreased from 1.0 to 0.51. SinceGa_(Y)In_(1−Y)P with a gallium composition ratio of 0.51±0.01 has alattice that roughly matches that of gallium arsenide (GaAs), even if acontact layer consisting of GaAs is stacked upon the Ga_(Y)In_(1−Y)Pelectron-supply layer, it has an advantage in that it is possible toprevent deterioration of the crystallinity of the electron-supply layerarising from mismatch of lattices.

[0071] In the fifth preferred embodiment according to claim 5 of thepresent invention, an n-type Ga_(Y)In_(1−Y)P layer wherein the galliumcomposition ratio (=Y) is constant with a thickness in the range from1-20 nanometers (units: nm) is provided in the region of the junctioninterface between the electron-supply layer and the channel layer. Byproviding a Ga_(Y)In_(1−Y)P layer with a constant gallium compositionratio (=Y) in contact with the Ga_(Z)In_(1−Z)As (0<Z≦1) channel layer,the height of the barrier with the channel layer can be homogeneouslystabilized. If the thickness of the aforementioned region with aconstant gallium composition ratio becomes excessively thick, problemsarising from mismatches with the lattice of the Ga_(Z)In_(1−Z)As (0<Z≦1)channel layer become conspicuous, and it becomes difficult to obtain aGa_(Y)In_(1−Y)P electron-supply layer with superior homogeneity of theindium (In) composition. In a typical electron-supply layer with athickness of 10 nm-40 nm, in order to obtain stably a Ga_(Y)In_(1−Y)Pelectron-supply layer with superior homogeneity of the indiumcomposition and with also a superior surface state and the like, theaforementioned thickness of the Ga_(Y)In_(1−Y)P layer with a constantgallium composition is preferably in the range 1-20 nm, more preferablyin the range 1-10 nm and most preferably in the range 1-5 nm. Note thatin an extremely thin film wherein the thickness of the Ga_(Y)In_(1−Y)Player with a constant gallium composition ratio (=Y) is extremely thinat less than 1 nm, because of instability in the control of the galliumcomposition (=Y), an enlarged junction barrier cannot be obtained stablyat the junction interface with the channel layer. The height of thejunction barrier can be measured by means of the capacitance-voltage(C-V) method that utilizes Schottky junction electrodes (see Appl. Phys.Lett., 43(1) (1983), p. 118).

[0072] The effect of the gradient pattern of the gallium compositionaccording to the fifth preferred embodiment upon the improvement in thesurface state of the Ga_(Y)In_(1−Y)P electron-supply layer is comparedagainst the prior art by means of the “haze” of the surface (see TakaoAbe, “Silicon Crystal Growth and Wafer Working,” (published by Baifukan,May 20, 1994, first edition), pp. 322-326). For example, while an n-typeGa_(0.51)In_(0.49)P electron-supply layer with a gallium compositionratio of 0.51 stacked upon an n-type Ga_(0.80)In_(0.20)As channel layerto a total thickness of 25 nm had a “haze” of the surface afterdeposition of 500-600 parts per million (ppm), if the fifth preferredembodiment is followed to make the gallium composition ratio (=Y) 1.0 ina 5-nm region from the junction interface with the Ga_(0.80)In_(0.20)Aschannel layer, and then it is decreased to 0.51 with the increase inthickness until a total thickness of 25 nm is reached, thus forming anelectron-supply layer constituting a Ga_(Y)In_(1−Y)Pgradient-composition layer, the “haze” of the surface after depositionwas improved to 50-60 parts per million (ppm).

[0073] In the sixth preferred embodiment according to claim 6 of thepresent invention, the Ga_(X)In_(1−X)P (0<X≦1) spacer layer may beformed by means of atmospheric-pressure or low-pressure MOCVD or othervapor-deposition means using trimethyl gallium (chemical formula:(CH₃)₃Ga) as the gallium (Ga) source, trimethyl indium (chemicalformula: (CH₃)₃In) as the indium (In) source and phosphine (chemicalformula: PH₃) as the phosphorus (P) source, for example. Triethylgallium (chemical formula: (C₂H₅)₃Ga) can also be used as the gallium(Ga) source. The Ga_(X)In_(1−X)P layer may be formed using a(CH₃)₃Ga/C₅H₅In/PH₃ MOCVD reaction using cyclopentadienyl indium(chemical formula: C₅H₅In) (see JP-B-8-17160) as the indium (In) source,for example. The Ga_(X)In_(1−X)P spacer layer (gradient-compositionlayer) with a gradient in the gallium composition such that the galliumcomposition ratio (X) decreases in the direction of increasing layerthickness from the junction interface with the Ga_(Z)In_(1−Z)As (0<Z≦1)channel layer can be formed by reducing the amount (concentration) ofthe gallium source provided to the MOCVD reaction system with increasingfilm formation time while maintaining a constant amount (concentration)of the indium source supplied to the MOCVD reaction system. In addition,film formation can be performed by increasing the amount of the indiumsource supplied together with increasing film formation time whilemaintaining a constant amount of the gallium source supplied to theMOCVD reaction system.

[0074] A Ga_(Y)In_(1−Y)P (0<Y≦1) electron-supply layer is provided incontact with the spacer layer. In consideration of matching the latticeof the GaAs substrate, the electron-supply layer should preferably havean indium composition ratio (1−Y) set to 0.49 (or more strictly 0.485)and a gallium composition ratio (Y) set to 0.51 as shown in thepreferred embodiment recited in claim 7.

[0075]FIG. 4 schematically illustrates the pattern of the gradient ingallium composition in the interior of the Ga_(X)In_(1—X)Pgradient-composition spacer layer. The gradient pattern in the galliumcomposition illustrated in FIG. 4 is one example of the compositiongradient patterns allowable by the present invention, and in thisfigure, (a) shows the change in the gallium composition when the galliumcomposition is varied uniformly and linearly together with the increasein the thickness of the spacer layer. The symbol (b) shows the gradientpattern in the case in which the gallium composition is kept constantfrom the junction interface with the channel layer and then the galliumcomposition is gradually reduced uniformly and linearly. For example, ina Ga_(X)In_(1−X)P spacer layer with a thickness of 7 nm, the galliumcomposition is kept constant in the region up until the thickness fromthe junction surface with the channel layer becomes 2 nm, and thereafterthere is a means of giving a gradient composition wherein the galliumcomposition is reduced. The symbol (c) shows an example in the case inwhich the gallium composition is reduced in a curved manner. Inaddition, the symbol (d) is a gradient pattern in the case in which thegallium composition is reduced in a step-wise manner. For example, thegallium composition ratio (=X) may be made 0.90 in the region whereinthe thickness from the junction interface with the channel layer is upto 2 nm, and next the gallium composition ratio can be made 0.70 inanother 2-nm thick region, and then another 2-nm thick region with agallium composition of 0.51 can be used to give a multi-layerconstitution of individual Ga_(Y)In_(1−Y)P layers, and by thus reducingthe gallium composition layer step-wise by 0.2, a gradient-compositionspacer layer can be formed.

[0076] The gradient pattern is not limited to the patterns illustratedin FIG. 4, but in any of the gradient patterns, as recited in thepreferred embodiments according to claims 8-10 of the present invention,the gallium composition ratio (=X) of the Ga_(X)In_(1−X)P spacer layerat the junction interface with the Ga_(Z)In_(1−Z)As (0<Z≦1) channellayer is preferably 0.70 or greater, more preferably 0.85 or greater,and even more preferably 1.0. This is because, by making the galliumcomposition ratio (=X) 0.70 or greater, the mobility of thetwo-dimensional electron gas accumulated in the channel layer can beincreased. In addition, the gallium composition ratio should preferablydecrease up to the vicinity of 0.51. This is because if lattice matchingwith the Ga_(0.51),In_(0.49)P forming the electron-supply layer isachieved, then a spacer layer with superior crystallinity suited toaccumulating the electrons supplied from the electron-supply layer as atwo-dimensional electron gas in the Ga_(Z)In_(1−Z)As (0<Z≦1) channellayer can be constituted.

[0077] Table 1 shows the mobility of a GaInP TEGFET which has aGa_(X)In_(1−X)P spacer layer with a gradient in the gallium composition(=X) according to the present invention, compared against that of atypical conventional GaInP TEGFET having a G_(0.51)In_(0.49)P layer witha gallium composition ratio of 0.51 as the spacer layer. TABLE 1 Sheetcarrier density Mobility Ga composition ratio in (Units: x 10¹² cm⁻²)(Units: x 10^(3 cm) ³/Vs) Type of GaInP Ga_(x)In_(1-x)P spacer layerRoom Room TEGFET (X) temperature 77 K temperature 77 K Prior art 0.511.9 1.6 4.2 14 Prior art 0.51 1.7 1.4 4.4 17 Present invention 0.75→0.511.8 1.5 5.5 21 Present invention 0.85→0.51 1.9 1.7 6.0 23

[0078] Among the TEGFETs according to the present invention listed inTable 1, regarding the gallium composition ratio of the spacer layer forexample, the notation “0.75→0.51” means that the gallium compositionratio of 0.75 at the junction interface with the channel layer isreduced to 0.51 at the junction interface with the electron-supplylayer. As shown in this table, with a GaInP TEGFET provided with agradient-composition spacer layer according to the present invention,even at roughly the same sheet carrier density, a mobility higher thanthat of the prior art is exhibited at both room temperature (300 Kelvin(K)) and the temperature of liquid nitrogen (77K). In passing, both themobility and sheet carrier density can be measured by the ordinary Halleffect measurement method. To wit, a Ga_(X)In_(1−X)P spacer layer with agradient in the gallium composition and a gallium composition ratio atthe junction interface with the carrier layer has the meritorious effectof exhibiting high mobility.

[0079] In particular, as shown in the ninth preferred embodimentaccording to claim 9 of the present invention, the Ga_(X)In_(1−X)Pgradient-composition spacer layer wherein the gallium composition ratio(=X) at the junction interface with the Ga_(Z)In_(1−Z)As (0<Z≦1) channellayer is set to 1.0, namely it is made to be gallium phosphide (chemicalformula: GaP), gives a particularly high mobility. Even in this case, itis preferable that the gallium composition ratio at the junctioninterface with the Ga_(0.51)In_(0.49)P electron-supply layer be 0.51. Towit, a Ga_(X)In_(1−X)P spacer layer which is preferable in the ninthpreferred embodiment is a crystal layer wherein the gallium compositionratio (=X) is decreased from 1.0 to 0.51 when going from the junctioninterface with the channel layer to the junction interface with theelectron-supply layer. A Ga_(X)In_(1−X)P spacer layer (X=1.0→0.51) withsuch a gradient composition is obtained by forming a GaP layer whilesupplying none of the indium source to the MOCVD reaction system at thestart of film formation and thereafter, gradually increasing the amountof the indium source supplied to the reaction system so that the galliumcomposition becomes 0.51.

[0080] In addition, in the eleventh preferred embodiment according toclaim 11 of the present invention, the n-type Ga_(X)In_(1−X)Pgradient-composition spacer layer is constituted from n-typeGa_(X)In_(1−X)P (0.51≦X≦1.0) doped with boron (element symbol: B). Theboron-doped Ga_(X)In_(1−X)P gradient-composition layer is formed with agradient in the gallium composition, and it can be formed whilesupplying the boron source into the MOCVD system. Examples of thesources of boron for doping include trimethylboron (chemical formula:(CH₃)₃B) and triethylboron (chemical formula: (C₂H₅)₃B). Boron ispreferably doped such that the boron atom density is 1×10¹⁶ atoms/cm⁻³or greater and 1×10¹⁸ atoms/cm⁻³ or less. Furthermore, the boron dopingshould preferably be performed to an atom density that exceeds theapproximate carrier density of the Ga_(X)In_(1−X)P gradient-compositionlayer. The boron atom density in the interior of the Ga_(X)In_(1−X)Pgradient-composition layer can be adjusted with the amount of the sourceof boron for doping supplied to the MOCVD reaction deposition system. Inaddition, the boron atom density (units: atoms/cm⁻³) in the interior ofthe Ga_(X)In_(1−X)P gradient-composition layer can be measured usingordinary secondary ion mass spectrometry (SIMS).

[0081] With boron doping, the carrier density of the Ga_(X)In_(1−X)Player which is a gradient-composition layer can be reduced. For example,the carrier density of the Ga_(X)In_(1−X)P gradient-composition layerwhich is approximately 5×10¹⁷ atoms/cm⁻³ in the undoped state can bereduced by one or more orders of magnitude by boron doping. To wit, thegradient-composition layer can be made a layer with a higher electricalresistance. Thereby, the two-dimensional electron gas accumulated withinthe Ga_(Z)In_(1−Z)As (0<Z≦1) channel layer can be reduced to the degreeof received ionization scattering, and therefore, since a high electronmobility becomes manifested, and a GaInP TEGFET with a superiortransconductance (g,) property can be provided.

[0082]FIG. 6 is a schematic cross section of the epitaxial stackingstructure 8A for conceptually explaining the twelfth preferredembodiment according to claim 12 of the present invention. In theworking of this embodiment, a semi-insulating GaAs single crystal with a{100} crystal plane as its primary plane can be used as the substrate801. A semi-insulating GaAs single crystal with a {100} plane as itsprimary plane which has a surface inclined by an angle of roughly ±10°in the [110] crystal direction from the {100} plane can also be used asthe substrate 801. In addition, a GaAs single crystal with aroom-temperature resistivity (specific resistance) of 10⁷ ohm-centimeter(units: Ω·cm) can be preferably used as the substrate 801.

[0083] Upon the surface of the substrate 801 is deposited a superlatticeperiod structure 802 a consisting preferably of an undopedAl_(L)Ga_(1−L)As (0≦L≦1) layer vapor-deposited by the MOCVD method usingtrimethyl gallium ((CH₃)₃Ga) or other trialkyl gallium compound as thegallium source, thus constituting one part 802 a of the buffer layer802. The methyl groups added to the trimethyl gallium compound becomethe source of carbon impurities admixed into the interior of theAl_(L)Ga_(1−L)As (0≦L≦1) layer, thereby electrically compensating forresidual donors within the layer, and having the meritorious effect ofgiving an Al_(L)Ga_(1−L)As (0≦L≦1) layer which has a high resistance inthe undoped state. Therefore, if a trimethyl gallium compound is used asthe starting material, a high-resistance buffer layer can be readilyconstituted. Even with a gallium compound which is a trialkyl galliumcompound with three added hydrocarbon groups wherein two added groupsare methyl groups, a similar meritorious effect may be obtained, but theefficacy is weaker than that of a trimethyl gallium compounds. In theevent that a diethyl methyl gallium compound is used as the galliumsource, for example, the efficacy of the adoption of high resistance dueto the electrical compensation effect of carbon impurities becomes evenweaker.

[0084] The superlattice structure 802 a is constituted by periodicalstacking of Al_(L)Ga_(1−L)As (0≦L≦1) layers with mutually differingaluminum composition ratios (=L). It can be constituted with aperiodically stacked structure of Al_(0.3)Ga_(0.7)As which has analuminum composition ratio of 0.3 and GaAs which has an aluminumcomposition ratio equivalent to 0, for example. In addition, it can beconstituted with a periodically stacked structure of Al_(0.1)Ga_(0.9)Asand aluminum arsenide (chemical formula: AlAs), for example. In aperiodically stacked structure with a multi-layer structure consistingof two layers with different aluminum composition ratios as a unit, theappropriate thickness of the constituent layers 802-1 and 802-2 is 10nanometers (units: nm) or greater and 100 nm or less. The number ofstacking periods is preferably 2 or greater and more preferably 5 orgreater. A high-resistance buffer layer consisting of a superlatticestructure with a heterojunction constitution constituting 5 or morestacking periods of multi-layer units consisting of Al_(L)Ga_(1−L)As(0≦L≦1) layers with different aluminum composition ratios has ameritorious effect of suppressing the propagation of dislocations or thelike from the substrate 801 to the channel layer 803 and other upperlayers, and thus gives the effect of providing a Ga_(Z)In_(1−Z)Aschannel layer 803 with a low crystalline defect density and highquality, and that has a superior surface flatness.

[0085] The Al_(L)Ga_(1−L)As (0≦L≦1) layers made from organic ethylcompounds as starting materials which constitute the other portions 802b of the buffer layer 802 provided joined to the superlattice structurecan be deposited using triethyl gallium (chemical formula: (C₂H₅)₃Ga)and triethyl aluminum (chemical formula: (C₂H₅)₃Al). In the case ofMOCVD deposition using ethyl compounds of Group III elements, the ethylgroups dissociated by thermal decomposition recombine and become ethane(molecular formula: C₂H₆) and other volatile components and areexhausted out of the chemical vapor deposition reaction system, so theamount of carbon impurities admixed into the interior of the crystallayer will not be as large as in the case of methyl compounds.Therefore, the resistance will not be as high as that of layersdeposited from methyl compounds as starting materials. However, by usingan Al_(L)Ga_(1−L)As (0≦L≦1) deposition layer 802 b deposited from ethylcompounds of Group III elements as starting materials, there is theeffect that indium-containing Group III-V compound semiconductor layerswith a homogeneous indium composition can be deposited. Since these areeasily dissociated ethyl groups, the probability that the surface of thedeposition layer will be covered with carbon-containing residues is low,so one reason for this is thought to be that a clean surface is exposed.

[0086] The Al_(L)Ga_(1−L)As (0≦L≦1) layer 802 b made from organic ethylcompounds as starting materials can be provided on any plane of thesuperlattice structure 802 a which constitutes the buffer layer 802. Forexample, it can be disposed between the semi-insulating GaAs substrate801 and the superlattice structure 802 a. In addition, it can bedisposed between the superlattice structure 802 a and theGa_(Z)In_(1−Z)As (0<Z≦1) channel layer 803. In addition, it can also beprovided on both sides of the superlattice structure 802 a. The effectof homogenizing the indium composition in the indium-containing GroupIII-V compound semiconductors 803 and 804 is greatest in the casewherein the Al_(L)Ga_(1−L)As (0≦L≦1) layer 802 b deposited using organicethyl compounds as starting materials is provided joined upon thesuperlattice structure 802 a. While there is also a method wherein theAl_(L)Ga_(1−L)As (0≦L≦1) layer 802 b which uses organic ethyl compoundsas starting materials is disposed as being joined to the surface of thesubstrate 801, the effect of homogenization of the indium composition islessened the further in distance from the of Ga_(Z)In_(1−Z)As channellayer 803 and the Ga_(L)In_(1−L)P electron-supply layer 805.

[0087] In the event that the Al_(L)Ga_(1−L)As (0≦L≦1) layer 802 b whichuses organic ethyl compounds as starting materials is provided only onthe upper surface of the superlattice structure 802 a, either theconstituent layer 802-1 or 802-2 is joined to the surface of thesemi-insulating GaAs substrate 801 below the substrate 801 side of thesuperlattice structure 802 a regardless of the aluminum compositionratio. The constituent layer (802-1 or 802-2) provided joined to thesurface of the semi-insulating GaAs substrate 801 (limited to onelayer), if its thickness is greater than that of the other constituentlayers, then it is also effective in the aforementioned homogenizationof the indium composition and moreover, gives rise to the effect of abuffer layer that suppresses changes in the crystalline quality of theupper layers due to fluctuations in the crystallographic specificationsof the substrate crystal.

[0088] The Al_(L)Ga_(1−L)As (0≦L≦1) layer (802-1 or 802-2) with acompensation ratio (=K) within the stipulated range which constitutesthe buffer layer 802 of the thirteenth preferred embodiment according toclaim 13 of the present invention can be deposited by adjusting theso-called V/III ratio. In an atmospheric-pressure or low-pressure MOCVDdeposition reaction system, the V/Ill ratio is defined to be the ratioof supply of, for example, arsine (chemical formula: AsH₃) (=V) totrimethyl gallium (=III) supplied to within the system (see ibid, J.Crystal Growth, 55 (1981)). As one example, in an AsH₃/(CH₃)₃Ga/hydrogen(H₂) low-pressure MOCVD system, under conditions of a depositiontemperature of 640° C., deposition pressure of 10⁴ Pascal (Pa),formation is possible with the V/III ratio (=AsH₃/(CH₃)₃Ga) in the rangefrom 7 or greater to 40 or less.

[0089] The compensation ratio (=K) can be calculated based on the donordensity (N_(d)) and the acceptor density (N_(a)). N_(d) and N_(a) can becalculated based on the Brooks-Herring formula from the values of thespecific resistance, mobility and carrier density which are measured bythe Hall effect method at the temperature of liquid nitrogen (77K), forexample, (see Phys. Rev., Vol. 164, No. 3 (1967), pp. 1025-1031). Withn-type Al_(L)Ga_(1−L)As (0≦L≦1,) in the state N_(d)≧N_(a), K is given byN_(a)/N_(d). With p-type Al_(L)Ga_(1−L)As wherein N_(a)>N_(d), K isgiven by N_(d)/N_(a). N-type or p-type Al_(L)Ga_(1−L)As (0≦L≦1) with theV/III ratio appropriately selected and the K value preferably in therange 0.9 or greater and 1.0 or less has a particularly high resistance.For example, an undoped GaAs layer deposited by a (CH₃)₃Ga/AsH₃/H₂ MOCVDmethod with a V/Ill ratio of 20 has a compensation ratio of 1.0, and itscarrier density is less than 5×10¹⁴ cm⁻³. Thus, such a high-resistancelayer has the effect of giving a buffer layer 802 a with ahigh-resistance superlattice structure which reduces the leakagecurrent.

[0090] In the fourteenth preferred embodiment according to claim 14 ofthe present invention, the superlattice structure 802 a is constitutedusing a p-type undoped GaAs layer obtained by setting the V/III ratio onthe relatively low side and which has a compensation ratio (K) in therange 0.9 or greater and 1.0 or less, and a carrier density 5×10¹⁵ cm⁻³or less as the constituent layer (e.g., 802-1). If p-type GaAs is used,electrons are caught by being bound to holes, and as a result it has theeffect of giving a buffer layer constituent layer able to cut off theleakage current. If the carrier (hole) density exceeds 1×10¹⁵ cm⁻³, thenpn junctions must be formed in other constituent layers of thesuperlattice structure (e.g., 802-2), and there are problems in thatthere are cases wherein the high-speed response of the TEGFET is lostdue to increased capacitance. In p-type GaAs with a hole density of1×10¹³ cm⁻³ or less, because of the low hole density within the layer,there are cases wherein sufficient numbers of electrons cannot becaptured, obstructing any further decrease in the leakage current.Therefore, with a p-type GaAs layer (e.g., 802-1) forming thesuperlattice structure 802 a, the preferable carrier density is 1×10¹³cm⁻³ or greater and 1×10¹⁵ cm⁻³ or less. More preferable is 5×10¹³ cm⁻³or greater and 1×10¹⁴ cm⁻³ or less.

[0091] Moreover, the buffer layer 802 is constituted with anAl_(L)Ga_(1−L)As (0≦L≦1) layer with a compensation ratio (K) in therange 0.9 or greater and 1.0 or less, and a carrier density 1×10¹⁵ cm⁻³or less as a separate constituent layer (e.g., 802-2). An aluminumcomposition ratio (=L) in the range of 0.15 or greater and 0.35 or lessis preferable for giving the aforementioned p-type GaAs layer (e.g.,802-1) and a superlattice structure 802 a with a low leakage current.More preferable is 0.20 or greater and 0.30 or less. An Al_(L)Ga_(1−L)As(0≦L≦1) layer having such a suitable aluminum composition ratio has aforbidden band between 0.2 electron-volt (units: eV) and 0.4 eV higherthan that of GaAs, so regardless of whether the conduction type isp-type, i-type (high-resistance type) or n-type, it has the effect ofreducing the leakage current, but in the case that the aforementionedp-type GaAs layer is to be the other constituent layer, it is preferablya p-type Al_(L)Ga_(1−L)As layer.

[0092] The superlattice structure 802 a and the Al_(L)Ga_(1−L)As (0≦L≦1)layer 802 b which uses organic ethyl compounds as starting materialsjoined thereto can be formed by the MOCVD method or the molecular beamepitaxy (MBE) method or other chemical vapor deposition method. Since achannel layer 803 consisting of a phosphorus-containing Group III-Vcompound semiconductor and an electron-supply layer 805 must bedeposited upon the buffer layer 802, the MOCVD method is preferablyused. Other means of forming the epitaxial stacking structure 8A forTEGFET applications using different deposition methods are conceivable,for example, forming the buffer layer 802 by MBE and forming the channellayer 803 and electron-supply layer 805 by MOCVD.

[0093] The fifteenth preferred embodiment according to claim 15 of thepresent invention is characterized in that the Ga_(Z)In_(1−Z)As (0<Z≦1)channel layer 803 is provided joined to the Al_(M)Ga_(1−M)As (0<M≦1)layer 802 b which forms part of the buffer layer 802 and which isvapor-deposited with organic ethyl compounds as starting materials. Byadopting a constitution wherein it is disposed directly below theindium-containing Group Ill-V compound semiconductor layer 803, it hasthe greatest effect in improving the homogeneity of the indiumcomposition. For example, while the homogeneity of the indiumcomposition ratio in the case of forming a channel layer 803 fromGa_(0.80)In_(0.20)As which has an indium composition ratio of 0.20 isroughly ±6% in the case wherein an undoped GaAs layer made fromtrimethyl gallium as its starting materials is used as the layer uponwhich deposition is performed, this less than ±2% with this preferredembodiment, and typically improved by ±1% or less. In addition, even inan Al_(M)Ga_(1−M)As layer using a (CH₃)₃Al/(C₂H₅)Ga starting materialsystem, this has the effect of increasing the homogeneity of the indiumcomposition ratio from roughly ±6% to roughly ±3%. The indiumcomposition ratio of the Ga_(Z)In_(1−Z)As channel layer or theGa_(L)In_(1−L)P electron-supply layer can be determined from the angleof diffraction found from ordinary x-ray diffraction methods, or fromthe photoluminescence (PL) light emission wavelength.

[0094] In particular, in the sixteenth preferred embodiment according toclaim 16 of the present invention, the Al_(M)Ga_(1−M)As (0≦M≦1) layer802 b vapor-deposited using organic ethyl compounds as startingmaterials is constituted from n-type undoped Al_(M)Ga_(1−M)As with acarrier density of 5×10¹⁵ cm⁻³ or less. The Al_(M)Ga_(1−M)As (0≦M≦1)layer 802 b with a carrier (electron) density preferably 5×10¹⁵ cm⁻³ orless has the effect of suppressing the leakage of the operating currentflowing in the Ga_(Z)In_(1−Z)As channel layer 803 into the interior ofthe buffer layer 802. While even a p-type undoped Al_(M)Ga_(1−M)As layerwould have the effect of reducing the leakage current into the bufferlayer 802, with a MOCVD method that uses organic ethyl compounds as thestarting materials, it is difficult to reduce the admixture of carboncompounds due to the effect of ethyl groups and stably obtain a p-typeAl_(M)Ga_(1−M)As layer in the undoped state. In addition, while a p-typeAl_(M)Ga_(1−M)As layer can be obtained by means of doping with p-typeimpurities, if the constitution has an Al_(M)Ga_(1−M)As layer with alarge total amount of impurities (=N_(d)+N_(a)) joined directly belowthe Ga_(Z)In_(1−Z)As channel layer 803, there is a problem in that thephotoresponsibility of the source-drain current (I_(ds)) becomes large.For this reason, an undoped n-type Al_(M)Ga_(1−M)As (0≦M≦1) layer ismost preferably used.

[0095] The Al_(M)Ga_(1−M)As (0≦M≦1) layer 802 b deposited using organicethyl compounds as described above has a low carbon impurity content andits resistance value is typically low compared to that of crystal layersmade from organic methyl compounds. Therefore, if the thickness of theAl_(M)Ga_(1−M)As layer deposited using organic ethyl compounds isexcessively large, there is a problem in that this invites a resultwherein the leakage current into the buffer layer 802 is increased.Thus, the thickness of the Al_(M)Ga_(1−M)As layer is preferably 100 nmor less. Control of the layer thickness is performed by controlling thefilm formation time. In particular, in the seventeenth preferredembodiment according to claim 17 of the present invention, the thicknessof the Al_(M)Ga_(1−M)As (0≦M≦1) layer 802 b vapor-deposited usingorganic ethyl compounds as starting materials is made to be no thickerthan either of the Al_(L)Ga_(1−L)As (0≦L≦1) layers (802-1 or 802-2) witha different aluminum composition ratio (L) vapor-deposited using organicmethyl compounds as starting materials which constitute the superlatticestructure 802 a. For example, joined to the superlattice structure 802 aconsisting of an Al_(L)Ga_(1−L)As layer with a thickness of 5 nm is anAl_(M)Ga_(1−M)As (0≦M≦1) layer 802 b deposited using organic ethylcompounds with a thickness of 50 nm or less. An n-type Al_(M)Ga_(1−M)As(0≦M≦1) layer 802 b with such a thickness has the effect of reducing thehysteresis of the I_(ds) and also exhibits the effect of increasing thestability of g_(m).

[0096] In the eighteenth preferred embodiment according to claim 18 ofthe present invention, the aluminum composition ratio (M) of the n-typeAl_(M)Ga_(1−M)As layer 802 b vapor-deposited using organic ethylcompounds as starting materials is set to be no greater than thealuminum composition ratio (L) of either of the Al_(L)Ga_(1−L)As (0≦L≦1)layers (802-1 or 802-2) which constitute the superlattice structure 802a. The aluminum composition (=M) is to be preferably no greater than 0.4so as not to give an indirect-transition type semiconductor. It ispreferably no greater than 0.3. The optimal aluminum composition ratiois 0, namely a GaAs constitution. The aluminum composition ratio (=M)can be controlled, for example, by adjusting the ratio of the amount of(C₂H₅)₃Al to the total amount triethyl aluminum (chemical formula:(C₂H₅)₃Al) and triethyl gallium (chemical formula: (C₂H₅)₃Ga) suppliedto the MOCVD deposition system. A buffer layer 802 to which an n-typeAl_(M)Ga_(1−M)As layer 802 b having such a suitable aluminum compositionratio is joined has the effect of providing a high electron mobilityfield effect transistor with low photoresponsibility and a small currentloop width in I_(ds).

[0097]FIG. 8 is a schematic cross section of the epitaxial stackingstructure 112A for conceptually explaining the nineteenth preferredembodiment according to claim 19 of the present invention. In theworking of this embodiment, a semi-insulating GaAs single crystal with a{100} crystal plane as its primary plane can be used as the substrate111. A semi-insulating GaAs single crystal with a {100} plane as itsprimary plane which has a surface inclined by an angle of roughly ±10°in the [110] crystal direction from the {100} plane can also be used asthe substrate 111. In addition, a GaAs single crystal with aroom-temperature resistivity (specific resistance) of 10⁷ ohm-centimeter(units: Ω·cm) can be preferably used as the substrate 111.

[0098] The buffer layer 112 upon the surface of the substrate 111 isconstituted from a superlattice period structure consisting preferablyof an undoped Al_(L)Ga_(1−L)As (0≦L≦1) layer vapor-deposited by theMOCVD method using trimethyl gallium ((CH₃)₃Ga) or other trialkylgallium compound as the gallium (Ga) source. The methyl groups added tothe trimethyl gallium compound become the source of carbon impuritiesadmixed into the interior of the Al_(L)Ga_(1−L)As (0≦L≦1) layer, therebyelectrically compensating for residual donors within the layer, andhaving the meritorious effect of giving an Al_(L)Ga_(1−L)As (0≦L≦1)layer which has a high resistance in the undoped state. Therefore, if atrimethyl gallium compound is used as the starting material, ahigh-resistance buffer layer can be readily constituted. Even with agallium compound which is a trialkyl gallium compound with three addedhydrocarbon groups wherein two added groups are methyl groups, a similarmeritorious effect may be obtained, but the efficacy is weaker than thatof a trimethyl gallium compounds. In the event that a diethyl methylgallium compound is used as the gallium source, for example, theefficacy of the adoption of high resistance due to the electricalcompensation effect of carbon impurities becomes even weaker.

[0099] The superlattice structure is constituted by stacking a repeatedpattern of Al_(L)Ga_(1−L)As (0≦L≦1) layers with mutually differingaluminum composition ratios (=L). It can be constituted with aperiodically stacked structure of Al_(0.3)Ga_(0.7)As which has analuminum composition ratio of 0.3 and GaAs which has an aluminumcomposition ratio equivalent to 0, for example. In addition, it can beconstituted with a periodically stacked structure of Al_(0.1)Ga_(0.9)Asand aluminum arsenide (chemical formula: AlAs), for example. In aperiodically stacked structure with a multi-layer structure consistingof two layers with different aluminum composition ratios, theappropriate thickness of the constituent layers 112-1 and 112-2 is 10nanometers (units: nm) or greater and 100 nm or less. The constituentlayers 112-1 and 112-2 are preferably high-resistance layers with acarrier density less than 5×10¹⁴ cm⁻³. The number of stacking periods ispreferably 2 or greater and more preferably 5 or greater. Ahigh-resistance buffer layer consisting of a superlattice structure witha heterojunction constitution constituting 5 or more stacking periods ofmulti-layer units consisting of Al_(L)Ga_(1−L)As (0≦L≦1) layers withdifferent aluminum composition ratios has a meritorious effect ofsuppressing the propagation of dislocations or the like from thesubstrate 111 to the channel layer 114 and other upper layers, and thusgives the effect of providing a Ga_(Z)In_(1−Z)As channel layer 114 witha low crystal defect density and high quality, and that has a superiorsurface flatness.

[0100] The GaAs layer made from triethyl gallium (chemical formula:(C₂H₅)₃Ga) as the starting material stacked upon the buffer layer 112constituting the superlattice structure can be deposited by the MOCVDmethod using a (C₂H₅)₃Ga/arsine (AsH₃)/hydrogen (H₂) reaction system. Byusing a GaAs deposition layer 113 which uses the ethyl compound(C₂H₅)₃Ga as the gallium source, there is the effect thatindium-containing Group III-V compound semiconductor layers with ahomogeneous indium composition can be deposited. The ethyl groupsdissociated by thermal decomposition recombine and become ethane(molecular formula: C₂H₆) and other volatile components and areexhausted out of the chemical vapor deposition reaction system, so theprobability that the surface of the deposition layer will be coveredwith carbon-containing residues is low, so one reason for this isthought to be that a clean surface is exposed.

[0101] If triethyl gallium is used as the starting material, the amountof carbon impurities admixed into the interior of the GaAs layer isreduced and the carrier density in the undoped state is typically higherthan that of a GaAs layer that uses trimethyl gallium as the startingmaterial. For example, when the ratio of the concentrations ofAsH₃/(CH₃)₃Ga supplied to a MOCVD reaction system (the so-called V/IIIratio) is set to the same 10.0, with trimethyl gallium, ahigh-resistance GaAs layer suitable for constituting the buffer layer112 with a p-type carrier density of 5×10¹³ cm⁻³ undoped isrecrystallized. In contrast, with triethyl gallium, a GaAs layer withn-type conductivity and a carrier density one order of magnitude greaterresults. If an extremely thick layer exhibiting such conductivity isdisposed directly below the Ga_(Z)In_(1−Z)As channel layer 114, theleakage current of the channel layer 114 only increases. Therefore, thethickness of the GaAs layer 113 made from triethyl gallium as itsstarting material is preferably a thickness between several nm androughly 100 nm. To give better results, the thickness of the GaAs layer113 should be made thinner the higher the carrier density. For example,for an n-type GaAs layer 113 with a carrier density of 1×10¹⁵ cm⁻³, themaximum preferable density is 30 nm.

[0102] Upon the GaAs layer 113 made from triethyl gallium as itsstarting material are successively deposited a Ga_(Z)In_(1−Z)As channellayer 114 and a Ga_(Y)In_(1−Y)P electron-supply layer 116. The GaAslayer 113 made from triethyl gallium as its starting material has theeffect of improving the homogeneity of the indium composition of theindium-containing Group III-V compound semiconductor layers forming theupper layers 114-116 to within ±2%. Indium-containing Group III-Vcompound semiconductor layers wherein the indium compositiondeteriorates to above ±2% become an obstacle to obtaining TEGFETs with ahomogeneous pinch-off voltage and transconductance (g_(m)). In addition,even in Al_(C)Ga_(1−C)As layers (0<C≦1) made from triethyl gallium astheir starting material, while they have the effect of makingindium-containing Group III-V compound semiconductor layers with upperlayers having superior homogeneity of the indium composition, ifaluminum (Al)-containing crystal layers are disposed, there is a problemin that photoresponsiveness in the drain current (see G. J. Ree, ed.,Semi-Insulating III-V Materials, (Shiva Pub. Ltd. (Kent, UK, 1980), pp.349-352)) and “hysteresis” of the source-drain current (see MakotoKikuchi, Yasuhiro Tarui, eds., “Illustrated Semiconductor Dictionary,”(Nikkan Kogyo Shimbunsha, Jan. 25, 1978), p. 238) and “kinks” readilyoccur (JP-A-10-247727 and JP-A-10-335350).

[0103] In the twentieth preferred embodiment according to claim 20 ofthe present invention, the channel layer 114 consisting ofGa_(Z)In_(1−Z)As with a small surface roughness described therein can beformed with the GaAs layer 113 in particular as the substrate layer, andusing a MOCVD method using a trimethyl compound in particular as thesource of the Group III constituent element. A MOCVD method using atrimethyl compound as the source of the Group III constituent element isdefined to have the meaning of an atmospheric-pressure or low-pressureMOCVD method using a trimethyl compound of at least one Group IIIelement of gallium or indium, for example, trimethyl gallium ((CH₃)₃Ga)as the gallium source, trimethyl indium (chemical formula: (CH₃)₃In) asthe indium source. In particular, cyclopentadienyl indium (chemicalformula: C₅H₅In) which has a bond valence of monovalent can be used.With a (CH₃)₃Ga/(CH₃)₃In/AsH₃/H₂ reaction system, upon the GaAs layer113 made of triethyl gallium as its starting material can be formed aGa_(Z)In_(1−Z)As layer 114 with a homogeneity in the indium compositionratio of ±1% or less. Homogeneity of the indium composition is definedto be given as the value found by dividing the difference between themaximum value and minimum value of the indium composition by a valuewhich is twice the average value of the indium composition. In a(C₂H₅)Ga/(CH₃)₃In/AsH₃/H₂ reaction system, the homogeneity of the indiumcomposition of the Ga_(Z)In_(1−Z)As layer is typically considered to bepoor at roughly ±6%.

[0104] In addition, by means of a MOCVD method using a trimethylcompound as the source of the Group III constituent element, upon theGaAs layer 113 made from triethyl gallium as its starting material isobtained a flat Ga_(Z)In_(1−Z)As layer which has a superior homogeneityof indium composition and also a low surface roughness due tosegregation of indium or the like. If the surface roughness is expressedin terms of haze (regarding haze, see Takao Abe, “Silicon Crystal Growthand Wafer Working,” (published by Baifukan, May 20, 1994, firstedition), pp. 322-326), then the GaAs layer 113 made from triethylgallium as its starting material also has the effect of reducing thehaze of the upper indium-containing Group III-V compound semiconductorlayers 114-116. The spacer layer 115 with a flat joining surface can bejoined upon the channel layer 114 which has a low surface roughness,namely little haze and thus its layer thickness has become homogeneous.If the joining surface is flat, then it has an advantage in that thetwo-dimensional electron gas can be localized in a region in thevicinity of the junction region of the channel layer 114. In order togive a heterojunction interface suited to the efficient localization ofa two-dimensional electron gas, the haze should be preferably 60 partsper million (ppm) or less. In a channel layer consisting of aGa_(Z)In_(1−Z)As layer having a surface roughness in excess of 60 ppm ashaze, the junction interface with the spacer layer lacks flatness andbecomes chaotic, so the electron mobility thus obtained also becomesheterogeneous, and as a result, GaInP TEGFETs with a hightransconductance (g_(m)) are not obtained.

[0105] In the twenty-first preferred embodiment according to claim 21 ofthe present invention, a spacer layer 115 is constituted from aGa_(X)In_(1−X)P (0<X≦1) layer formed by means of a MOCVD method using atrimethyl compound as the source of the Group III constituent element.As described above, upon the GaAs layer 113 made from triethyl galliumas its starting material can be constituted a channel layer 114consisting of a Ga_(Z)In_(1−Z)As layer which has a superior homogeneityof indium composition. Upon the channel layer 114 which has a homogenousindium composition can be stacked a Ga_(X)In_(1−X)P (0<X≦1) spacer layer115 which has a superior homogeneity of indium composition. Moreover,with a low-pressure or atmospheric-pressure MOCVD method based on a(CH₃)₃Ga/(CH₃)₃In/AsH₃ reaction system, a Ga_(X)In_(1−X)P (0<X≦1) layerwith even more superior homogeneity can be obtained. A Ga_(X)In_(1−X)Player with a homogeneity in the indium composition ratio of less than±1% is well suited to practical use as a spacer layer.

[0106] In addition, with a low-pressure or atmospheric-pressure MOCVDmethod based on a (CH₃)₃Ga starting material system, in addition to thehomogeneity of the indium composition ratio, a spacer layer 115 with aneven more superior surface flatness can be provided. For example, with a(CH₃)₃Ga/(CH₃)₃In/AsH₃/H₂ reaction system, at the time of deposition ofthe spacer layer 115, the haze at the surface of the spacer layer 115can be made 100 ppm or less, so a spacer layer 115 which can join to theelectron-supply layer 116 with a flat junction surface is provided. Ifthe haze of the surface of the Ga_(X)In_(1−X)P spacer layer 115 exceeds100 ppm, then differences in the thickness of the spacer layer 115 dueto regions lacking in surface flatness become conspicuous. For thisreason, the distance by which the channel layer 114 and electron-supplylayer 116 are spatially separated becomes different depending on theregion, so the degree of ionization scattering received by thetwo-dimensional electron gas within the channel layer 114 becomesheterogeneous. Therefore, a problem occurs in that the mobility of thetwo-dimensional electron gas obtained changes depending on the region.

[0107] The carrier density in the Ga_(X)In_(1−X)P (0<X≦1) layerconstituting the spacer layer 115 is preferably less than 1×10¹⁶ cm⁻³.The lower the carrier density the better, and depending on the case,even high-resistance is not a problem. The conduction type of the spacerlayer 115 is preferably n-type. A thickness of between 1 nm and 15 nm istypically suitable. As the thickness of the spacer layer 115 becomesthicker, the electron mobility exhibited by the two-dimensional electrongas increases, but conversely, the sheet carrier density decreases. Fora Ga_(Y)In_(1−Y)P electron-supply layer with a carrier density of 2×10¹⁸cm⁻³, a layer thickness that gives a sheet carrier density of 1.5×10¹²cm⁻² is preferable. The sheet carrier density is found by the ordinaryHall effect measurement method.

[0108] In the twenty-second preferred embodiment according to claim 22of the present invention, an electron-supply layer 116 is constitutedfrom an n-type Ga_(Y)In_(1−Y)P (0<Y≦1) layer with a surface haze of 200ppm or less. A Ga_(Y)In_(1−Y)P layer having such a surface roughness canbe formed from trimethyl gallium ((CH₃)₃Ga) or trimethyl indium((CH₃)₃In) as the Group III constituent starting material, disposed upona lower layer of a GaAs layer 113 made from triethyl gallium as itsstarting material. By using a reaction system which uses trimethylcompounds for both the gallium source and the indium source, aGa_(Y)In_(1−Y)P layer with an even lower surface roughness can beobtained even more stably. Haze can be measured by measuring theintensity of scattering of incident laser light or other means. Thethickness of the electron-supply layer 116 should be 20-40 nm.

[0109] The electron-supply layer 116 is preferably constituted fromGa_(Y)In_(1−Y)P (0<Y≦1) doped with n-type impurities. A particularlypreferable electron-supply layer 116 can be constituted from aGa_(0.51)In_(0.49)P crystal layer with an indium composition ratio(=1−Y) of 0.49. Since Ga_(0.51)In_(0.49)P matches the lattice of GaAs, aGaAs contact layer with few crystal defects arising from latticemismatching can be constituted as the upper layer. Suitable n-typeimpurities for doping into Ga_(0.51)In_(0.49)P include silicon (elementsymbol: Si) which has a small diffusion coefficient. The carrier densityof the Ga_(0.51)In_(0.49)P electron-supply layer 116 is preferably2-3×10¹⁸ cm⁻³. The carrier density can be measured by means of theordinary capacitance-voltage (C-V) method. The Ga_(Y)In_(1−Y)Pelectron-supply layer with a low surface roughness and superiorhomogeneity of indium composition has a superior homogeneity of carrierdensity, so it also has the effect of homogenizing the sheet carrierdensity mainly involving the two-dimensional electron gas.

[0110] In the twenty-third preferred embodiment according to claim 23 ofthe present invention, at the time of formation of the indium-containingGroup III-V compound semiconductor layers with superior surface haze bymeans of metal-organic chemical vapor deposition, cyclopentadienylindium (chemical formula: C₅H₅In(I)) which has a bond valence ofmonovalent is used as the indium source (see J. Electron. Mater., 25(3)(1996), pp. 407-409). Since C₅H₅In(I) exhibits the properties of a Lewisbase, the polymerization reaction with arsine (chemical formula: AsH₃)or phosphine (chemical formula: PH₃) as the representative source ofGroup V elements can be suppressed within the chemical vapor depositionenvironment (see J. Crystal Growth, 107 (1991), pp. 360-354). For thisreason, since the occurrence of organic indium-phosphorus polymers, forexample, is suppressed (see J. Chem. Soc., [1951] (1951), pp.2⁰⁰³-2013), the homogeneity of the indium composition is superior, andso it is essentially superior in obtaining indium-containing Group III-Vcompound semiconductor vapor-deposited layers.

[0111] In addition, C₅H₅In(I) has a lower vapor pressure (sublimationpressure) than that of trimethyl indium ((CH₃)₃In) and its filmformation rate is lower, so it is particularly suited to the formationof the Ga_(Z)In_(1−Z)As channel layer 114, Ga_(X)In_(1−X)P spacer layer115, electron-supply layer 116 and other and thin-film layers. In orderto induce a sublimation pressure suited to thin-film formation, theC₅H₅In(I) should preferably be kept in an approximate temperature rangefrom 40° C. to 70° C. An example of a companion gas accompanying thevapor of sublimed C₅H₅In(I) is hydrogen.

[0112] The twenty-seventh preferred embodiment is related particularlyto a high-electron-mobility field effect transistor fabricated using theaforementioned GaInP epitaxial stacking structure.

[0113] The above is a description of the preferred embodiments of thepresent invention and here follows a more detailed description of thepresent invention by means of working examples, but the presentinvention is in no way limited to these working examples.

WORKING EXAMPLE 1

[0114] In this working example, the present invention is described indetail using the case of constituting a GaInP two-dimensional electrongas field effect transistor by means of the MOCVD method as an example.FIG. 3 is a schematic cross section of a TEGFET 300 according to thisworking example.

[0115] The epitaxial stacking structure 3A for a TEGFET 300 applicationuses an undoped semi-insulating (100) 2° off GaAs single crystal as asubstrate 301. The specific resistance of the GaAs single crystal usedas the substrate 301 is 3×10⁷ Ω·cm. Upon the surface of the substrate301 with a diameter of 100 mm is deposited an Al_(L)Ga_(1−L)As/GaAssuperlattice structure as a constituent part 302-1 of the first bufferlayer constituting the buffer layer 302. The superlattice structure302-1 consists of an undoped Al_(0.30)Ga_(0.70)As layer 302 a with analuminum composition ratio (=L) of 0.30 and an undoped p-type GaAs layer302 b. The carrier density of the Al_(0.30)Ga_(0.70)As layer 302 a is1×10¹⁴ cm⁻³ and its thickness is 45 nm. The carrier density of thep-type GaAs layer 302 b is 7×10¹³ cm⁻³ and its thickness is 50 nm. Thenumber of stacking periods of the Al_(0.30)Ga_(0.70)As layer 302 a andp-type GaAs layer 302 b is 5 periods. The Al_(0.30)Ga_(0.70)As layer 302a and the p-type GaAs layer 302 b were all formed at 640° C. by means ofthe low-pressure MOCVD method based on a (CH₃)₃Ga/(CH₃)₃Al/AsH₃/H₂reaction system. The pressure at the time of film formation was 1.3×10⁴Pascal (Pa). Hydrogen was used as the carrier (transport) gas.

[0116] Upon the constituent part 302-1 of the first buffer layer 302 isstacked a GaAs layer 302 c deposited by means of a (C₂H₅)₃Ga/AsH₃/H₂reaction system low-pressure MOCVD method using triethyl gallium((C₂H₅)₃Ga) as the gallium (Ga) source, forming a second buffer layerconstituent part 302-2. The film formation temperature was 640° C. andthe pressure at the time of formation was 1.3×10⁴ Pa. The carrierdensity of the undoped n-type GaAs layer 302 c is 2×10¹⁵ cm⁻³ and itsthickness is 20 nm.

[0117] Upon the second buffer layer constituent part 302-2 is stacked anundoped n-type Ga_(0.80)In_(0.20)As layer deposited by means of alow-pressure MOCVD method using a (CH₃)₃Ga/C₅H₅In/AsH₃₁H₂ reactionsystem as a channel layer 303. The carrier density of theGa_(0.80)In_(0.20)As layer constituting the channel layer 303 is 1×10¹⁵cm⁻³ and its thickness is 13 nm. The homogeneity of the indiumcomposition ratio was found to be 0.20 (±0.4%) from the homogeneity ofthe photoluminescence (PL) wavelength. The haze value of the surface ofthis layer 303 measured from the intensity of scattering of incidentlaser light was found to be 12 ppm.

[0118] Upon the Ga_(0.80)In_(0.20)As channel layer 303 is stacked anelectron-supply layer 304 consisting of a silicon (Si) doped n-typeGa_(0.51)In_(0.49)P deposited by means of a low-pressure MOCVD methodusing a (CH₃)₃Ga/C₅H₅In/PH₃/H₂ reaction system with a gradientcomposition in the gallium composition ratio (=Y). The galliumcomposition ratio (=Y) of the electron-supply layer 304 at the junctioninterface 304 a with the undoped n-type Ga_(0.80)In_(0.20)As channellayer 303 was set to 0.88. The gallium composition ratio (=Y) of theelectron-supply layer 304 at the junction interface 304 b with then-type GaAs contact layer 305 was set to 0.51. The gradient in thisgallium composition ratio was given by uniformly and linearly reducingthe ratio of C₅H₅In to (CH₃)₃Ga (=C₅H₅1n/(CH₃)₃Ga) supplied to the MOCVDreaction system over time during the film-formation period fordepositing the electron-supply layer 304 to a thickness of 25 nm. Ahydrogen-disilane (Si₂H₆) gas mixture (concentration of 10 ppm byvolume) was used as the source of Si for doping. The carrier density ofthe electron-supply layer 304 is 2×10¹⁸ cm⁻³ and its thickness is 25 nm.The homogeneity of the indium composition ratio was found to be 0.51(10.5%) from the homogeneity of the photoluminescence (PL) wavelength.The haze value after stacking this layer 304 was found to be 18 ppm.

[0119] Upon the surface of the electron-supply layer 304 consisting ofn-type Ga_(0.51)In_(0.49)P is stacked a contact layer 305 consisting ofSi-doped n-type GaAs by means of a (CH₃)₃Ga/AsH₃/H₂ reaction system. Theaforementioned hydrogen-disilane (Si₂H₆) gas mixture was used as thesource of Si for doping. The carrier density of the n-type GaAs contactlayer 305 is 2×10¹⁸ cm⁻³ and its thickness is 100 nm. The haze of thesurface of the n-type GaAs contact layer 305 was measured to be 23 ppm.After the completion of epitaxial deposition of the constituent layers303-305 forming the epitaxial stacking structure 3A, it was heated to500° C. in an arsine (AsH₃)-containing atmosphere, and thereafter cooledto room temperature in a hydrogen atmosphere.

[0120] An ohmic electrode consisting of an indium-tin (In—Sn) alloy wasformed on the surface of the n-type GaAs contact layer 305 which formsthe uppermost surface layer of the epitaxial stacking structure 3A.Next, the ordinary Hall effect measurement method was used to measurethe electron mobility of the two-dimensional electron gas which travelsthrough the two-dimensional electron gas channel layer 303. The sheetcarrier density (n_(s)) at room temperature (300 Kelvin (K)) was1.6×10¹² cm⁻², and the average electron mobility (μ_(RT)) was 5800 (±2%)(cm²/V·s). In addition, the n_(s) at the temperature of liquid nitrogen(77K) was 1.5×10¹² cm⁻², and μ was 22,000 cm²/V·s, so a high electronmobility was exhibited.

[0121] After cooling, a patterning method employing knownphotolithography technology was used to create a recess in the surfaceof the n-type GaAs contact layer 305 which forms the surface-most layerof the epitaxial stacking structure 3A. Upon the n-type GaAs contactlayer 305 which remained as a mesa were formed a source electrode 306and a drain electrode 307. The source and drain ohmic electrodes 306 and307 were given a multi-layer structure consisting of gold-germanium (93%Au, 7% Ge by weight), nickel (Ni) and gold (Au) layers. The distancebetween the source electrode 306 and the drain electrode 307 was 10 μm.

[0122] Upon the surface of the Ga_(0.51)In_(0.49)P electron-supply layer304 exposed in the recess was formed a Schottky junction type gateelectrode 308 with a titanium (Ti) lower layer and an aluminum (Al)upper layer. The so-called gate length of the gate electrode 308 was 2μm.

[0123] The DC characteristics of the GaInP TEGF 300 were evaluated. Thesaturated source-drain current (I_(dss)) when a source/drain voltage of3 Volts (V) was applied was found to be 70 milliampere (mA). When thedrain voltage was swept from 0 V to 5 V, virtually no looping(hysteresis) in the drain current was observed. The room-temperaturetransconductance (g_(m)) measured at a source/drain voltage of 3.0 V washigh and homogeneous at 160±5 millisiemens (mS)/mm. In addition, theleakage current flowing between the Au—Ge ohmic electrodes formed at aninterval of 100 μm exposed upon the surface of the buffer layer 302 wasfound to be less than 1 μA at 40 V, thus exhibiting high breakdownresistance. For this reason, the drain current pinch-off voltage became2.38±0.03 V, so a GaInP TEGFET with a homogeneous threshold voltage wasobtained.

WORKING EXAMPLE 2

[0124] In this working example, the present invention is described indetail using as an example the case of constituting a GaInPtwo-dimensional electron gas field effect transistor (TEGFET) which hasa Ga_(Y)In_(1−Y)P gradient-composition layer of a type different thanthat in Working Example 1.

[0125] The TEGFET of this working example differs in constitution fromthat of Working Example 1 only in the constitution of theGa_(Y)In_(1−Y)P gradient-composition layer; otherwise it has theepitaxial stacking structure illustrated in FIG. 3 using the sameepitaxial constituent layers as those of Working Example 1. Thus, herefollows a description of this working example made with reference toFIG. 3. In this working example, the electron-supply layer 304 upon theGa_(0.80)In_(0.20)As channel layer 303 is constituted as aGa_(Y)In_(1−Y)P gradient-composition layer with a gradient in thegallium composition ratio such that the gallium composition ratio is 1.0at the junction interface 304 a of the electron-supply layer 304 withthe channel layer 303 and 0.51 at the junction interface 304 b with then-type GaAs contact layer 305. The thickness of the Ga_(Y)In_(1−Y)Pgradient-composition electron-supply layer 304 is 25 nm. In theelectron-supply layer 304 with an overall thickness of 25 nm, the regionwherein the thickness from the junction interface with the channel layeris up to 2 nm consists of Ga_(Y)In_(1−Y)P wherein the galliumcomposition ratio (=Y) is set to 1.0, so it is namely GaP. Thereafter,the gallium composition ratio is reduced uniformly and linearly with thepassage of time until the thickness of the electron-supply layer 304becomes 25 nm. Thereby the gallium composition ratio at the junctioninterface 304 b with the n-type GaAs contact layer 305 was made to be0.51. The gradient in the gallium composition ratio in theGa_(Y)In_(1−Y)P layer constituting the electron-supply layer 304 in thisworking example was given by uniformly and linearly increasing theamount of C₅H₅In supplied to the MOCVD reaction system during thedeposition period when the thickness of the electron-supply layer 304 isfrom 2 nm to 25 nm, while keeping constant the amount of (CH₃)₃Gasupplied to the MOCVD system. Upon the surface of the Ga_(Y)In_(1−Y)Pelectron-supply layer 304 is stacked the same n-type GaAs contact layer305 as in Working Example 1 to form a GaInP epitaxial stackingstructure. The sheet carrier density (n_(s)) at room temperature (300K)measured by means of the ordinary Hall effect measurement method was1.7×10¹² cm⁻², and the average electron mobility (μ_(RT)) was 5900 (±3%)(cm²/V·s). In addition, the n_(s) at the temperature of liquid nitrogen(77K) was 1.6×10¹² cm⁻², and μ at 77K was 22,700 cm²/V·s, so the GaInPepitaxial stacking structure provided with the electron-supply layer 304according to this working example also exhibited a high electronmobility. In addition, virtually no hysteresis (looping) was found inthe drain current of a GaInP TEGFET constituted using the sametechniques as those recited in Working Example 1. In addition, theroom-temperature transconductance (g_(m)) measured at a source/drainvoltage of 3.0 V was high and homogeneous at 165±5 millisiemens (mS)/mm.

WORKING EXAMPLE 3

[0126] In this working example, the present invention is described indetail using the case of constituting a GaInP two-dimensional electrongas field effect transistor (TEGFET) provided with a spacer layerconsisting of a Ga_(X)In_(1−X)P gradient-composition layer as anexample. FIG. 5 is a schematic cross section of a TEGFET 600 accordingto this working example.

[0127] The epitaxial stacking structure 6A for a TEGFET 600 applicationuses an undoped semi-insulating (100) 2° off GaAs single crystal as asubstrate 601. The specific resistance of the GaAs single crystal usedas the substrate 601 is 3×10⁷ Ω·cm. Upon the surface of the substrate601 with a diameter of 100 mm is deposited an Al_(L)Ga_(1−L)As/GaAssuperlattice structure which constitutes the buffer layer 602. Thesuperlattice structure consists of an undoped Al_(0.30)Ga_(0.70)As layerwith an aluminum composition ratio (=L) of 0.30 and an undoped p-typeGaAs layer. The carrier density of the Al_(0.30)Ga_(0.70)As layer is1×10¹⁴ cm⁻³ and its thickness is 45 nm. The carrier density of thep-type GaAs layer is 7×10¹³ cm⁻³ and its thickness is 50 nm. The numberof stacking periods of the Al_(0.30)Ga_(0.70)As layer and p-type GaAslayer is 5 periods. The Al_(0.30)Ga_(0.70)As layer and the p-type GaAslayer were all formed at 640° C. by means of the low-pressure MOCVDmethod based on a (CH₃)₃Ga/(CH₃)₃Al/AsH₃/H₂ reaction system. Thepressure at the time of film formation was 1.3×10⁴ Pascal (Pa). Hydrogenwas used as the carrier (transport) gas.

[0128] Upon the buffer layer 602 is stacked an undoped n-typeGa_(0.80)In_(0.20)As layer deposited by means of a low-pressure MOCVDmethod using a (CH₃)₃Ga/C₅H₅In/AsH₃/H₂ reaction system, forming thechannel layer 603. The carrier density of the Ga_(0.80)In_(0.20)As layerconstituting the channel layer 603 is 1×10¹⁵ cm⁻³ and its thickness is13 nm.

[0129] Upon the Ga_(0.80)In_(0.20)As channel layer 603 is stacked aspacer layer 604 consisting of undoped n-type Ga_(X)In_(1−X)P with agradient in the gallium composition (=X) deposited by means of alow-pressure MOCVD method using a (CH₃)₃Ga/C₅H₅In/PH₃/H₂ reactionsystem. The gallium composition ratio (=X) of the spacer layer 604 atthe junction interface 604 a with the undoped n-typeGa_(0.80)In_(0.20)As channel layer 603 was set to 0.88. The galliumcomposition ratio (=X) of the spacer layer 604 at the junction interface604 b with the Ga_(0.51)In_(0.49)P electron-supply layer 605 was set to0.51. The gradient in this gallium composition ratio was given byuniformly and linearly reducing the ratio of C₅H₅In to (CH₃)₃Ga(=C₅H₅In/(CH₃)₃Ga) supplied to the MOCVD reaction system over timeduring the film-formation period for depositing the electron-supplylayer 604 to a thickness of 6 nm.

[0130] Upon the Ga_(X)In_(1−X)P spacer layer 604 is stacked anelectron-supply layer 605 consisting of silicon (Si) doped n-typeGa_(0.54)In_(0.49)P deposited by means of a low-pressure MOCVD methodusing a (CH₃)₃Ga/C₅H₅In/PH₃/H₂ reaction system. A hydrogen-disilane(Si₂H₆) gas mixture (concentration of 10 ppm by volume) was used as thesource of Si for doping. The pressure at the time of film formation was1.3×10⁴ Pascal (Pa). The carrier density of the electron-supply layer605 is 2×10¹⁸ cm⁻³ and its thickness is 25 nm.

[0131] Upon the surface of the electron-supply layer 605 consisting ofn-type Ga_(0.51)In_(0.49)P is stacked a contact layer 606 consisting ofSi-doped n-type GaAs by means of a (CH₃)₃Ga/AsH₃/H₂ reaction system. Theaforementioned hydrogen-disilane (Si₂H₃) gas mixture was used as thesource of Si for doping. The carrier density of the n-type GaAs contactlayer 606 is 2×10¹⁸ cm⁻³ and its thickness is 100 nm. After thecompletion of epitaxial deposition of the constituent layers 603-606forming the epitaxial stacking structure 6A, it was heated to 500° C. inan arsine (AsH₃)-containing atmosphere, and thereafter cooled to roomtemperature in a hydrogen atmosphere.

[0132] An ohmic electrode consisting of an indium-tin (In—Sn) alloy wasformed on the surface of the n-type GaAs contact layer 606 which formsthe surface-most layer of the epitaxial stacking structure 6A. Next, theordinary Hall effect measurement method was used to measure the electronmobility for the two-dimensional electron gas which travels through thetwo-dimensional electron gas channel layer 603. The sheet carrierdensity (n_(s)) at room temperature (300 Kelvin (K)) was 1.6×10¹² cm⁻²,and the average electron mobility (μ_(RT)) was 6100±2% (cm²/V·s). Inaddition, the n_(s) at the temperature of liquid nitrogen (77K) was1.5×10¹² cm⁻², and μ was 23,000 cm²/V·s, so a high electron mobility wasexhibited.

[0133] After cooling, a patterning method employing knownphotolithography technique was used to create a recess in the surface ofthe n-type GaAs contact layer 606 which forms the uppermost surfacelayer of the epitaxial stacking structure 6A. Upon the n-type GaAscontact layer 606 which remained as a mesa were formed a sourceelectrode 607 and a drain electrode 608. The source and drain ohmicelectrodes 607 and 608 were given a multi-layer structure consisting ofgold-germanium (93% Au, 7% Ge by weight), nickel (Ni) and gold (Au)layers. The distance between the source electrode 607 and the drainelectrode 608 was 10 μm.

[0134] Upon the surface of the Ga_(0.51)In_(0.49)P electron-supply layer605 exposed in the recess was formed a Schottky junction type gateelectrode 609 with a titanium (Ti) lower layer and an aluminum (Al)upper layer. The so-called gate length of the gate electrode 609 was 1μm.

[0135] The DC characteristics of the GaInP TEGFET 600 were evaluated.The saturated source-drain current (I_(dss)) when a source/drain voltageof 3 Volts (V) was applied was found to be 68 milliampere (mA). When thedrain voltage was swept from 0 V to 5 V, virtually no looping(hysteresis) in the drain current was observed. The room-temperaturetransconductance (g_(m)) measured at a source/drain voltage of 3.0 V washigh and homogeneous at 160±5 millisiemens (mS)/mm. In addition, theleakage current flowing between the Au—Ge ohmic electrodes formed at aninterval of 100 μm exposed upon the surface of the buffer layer 602 wasfound to be less than 1 μA at 40 V, thus exhibiting high breakdownresistance. For this reason, the drain current pinch-off voltage became2.35±0.03 V, so a GaInP TEGFET with a homogeneous threshold voltage wasobtained.

WORKING EXAMPLE 4

[0136] In this working example, the present invention is describeddetail using as an example the case of constituting a GaInPtwo-dimensional electron gas field effect transistor (TEGFET) which hasa Ga_(X)In_(1−X)P gradient-composition layer of a type different thanthat in Working Example 3.

[0137] The TEGFET of this working example differs in constitution fromthat of Working Example 3 only in the constitution of theGa_(X)In_(1−X)P gradient-composition layer; otherwise it has theepitaxial stacking structure illustrated in FIG. 5 using the sameepitaxial constituent layers as those of Working Example 3. Thus, herefollows a description of this working example made with reference toFIG. 5. In this working example, the spacer layer 604 upon theGa_(0.80)In_(0.20)As channel layer 603 is constituted as aGa_(X)In_(1−X)P gradient-composition layer with a gradient in thegallium composition ratio such that the gallium composition ratio is 1.0at the junction interface 604 a of the channel layer 603 with the spacerlayer 604 and 0.51 at the junction interface 604 b with theGa_(0.51)In_(0.49)P electron-supply layer 605. The thickness of theGa_(X)In_(1−X)P gradient-composition electron-supply layer 604 is 8 nm.In the spacer layer 604 with an overall thickness of 8 nm, the regionwherein the thickness from the junction interface 604 a with the channellayer 603 is up to 2 nm consists of Ga_(X)In_(1−X)P wherein the galliumcomposition ratio (=X) is set to 1.0, so it is namely GaP. Thereafter,the gallium composition ratio is reduced uniformly and linearly with thepassage of time during the deposition period until the layer thicknessreaching 8 nm which is the total thickness of the spacer layer 604.Thereby the gallium composition ratio at the junction interface 604 bwith the Ga_(X)In_(1−X)P electron-supply layer 605 was made to be 0.51.The gradient in the gallium composition (=X) in the Ga_(X)In_(1−X)Player constituting the spacer layer 604 in this working example wasgiven by uniformly and linearly increasing the amount of C₅H₅In suppliedto the MOCVD reaction system during the deposition period when thethickness of the spacer layer 604 is from 2 nm to 8 nm, while keepingconstant the amount of (CH₃)₃Ga supplied to the MOCVD system.

[0138] Upon the surface of the Ga_(X)In_(1−X)P spacer layer 604 isstacked the same n-type Ga_(0.51)In_(0.49)P electron-supply layer 605and GaAs contact layer 606 as in Working Example 1 to form a GaInPepitaxial stacking structure. The sheet carrier density (n,) at roomtemperature (300K) measured by means of the ordinary Hall effectmeasurement method was 1.7×10¹² cm⁻², and the average electron mobility(μ_(RT)) was 6250±3% (cm²/V·s). In addition, the n_(s) at thetemperature of liquid nitrogen (77K) was 1.6×10¹² cm⁻², and μ at 77K was23,500 cm²/V·s, so the GaInP epitaxial stacking structure provided withthe spacer layer 604 according to this working example also exhibited ahigh electron mobility. In addition, virtually no hysteresis (looping)was found in the drain current of a GaInP TEGFET constituted using thesame techniques as those recited in Working Example 1. In addition, theroom-temperature transconductance (g_(m)) measured at a source/drainvoltage of 3.0 V was high and homogeneous at 165±5 millisiemens (mS)/mm.

WORKING EXAMPLE 5

[0139] In this working example, the present invention is described usingas an example the case of constituting a GaInP two-dimensional electrongas field effect transistor (TEGFET) which has the same gradientcomposition as that of Working Example 3 and which also has aGa_(X)In_(1−X)P (X=0.88→0.51) gradient-composition layer doped withboron (element symbol: B) as a spacer layer.

[0140] The TEGFET of this working example differs in constitution fromthat of Working Example 3 only in the constitution of theGa_(X)In_(1−X)P gradient-composition layer; the other epitaxialconstituent layers are the same as those of Working Example 3 so thedescription of this working example is made with reference to FIG. 5.

[0141] In this working example, boron doping is performed only duringthe period of deposition of the spacer layer 604 described in WorkingExample 3 upon the Ga_(0.80)In_(0.20)As channel layer 603. Thereby, aboron-doped Ga_(X)In_(1−X)P (X=0.88→0.51) spacer layer 604 is formedwith the gallium composition ratio (=X) set to 0.88 at the junctioninterface 604 a with the channel layer 603 and set to 0.51 at thejunction interface 604 b with the Ga_(0.51)In_(0.49)P electron-supplylayer 605. Commercial electronics-grade triethylboron ((C₂H₅)₃B) wasused as the source of boron for doping. In consideration of the factthat the carrier density of the n-type Ga_(X)In_(1−X)Pgradient-composition layer (X=0.88→0.51) constituting the spacer layer604 is roughly 1×10¹⁷ cm⁻³, the amount of triethylboron added (doped) tothe MOCVD reaction system is set such that the boron atom densitybecomes 3×10¹⁷ cm⁻³ in the interior of this gradient-composition layer.The carrier density of the Ga_(X)In_(1−X)P gradient-composition spacerlayer 604 was lowered to below 1×10¹⁶ cm⁻³ by means of the doping ofboron according to this working example.

[0142] Upon the Ga_(X)In_(1−X)P spacer layer 604 is stacked the samen-type Ga_(0.51)In_(0.49)P electron-supply layer 605 and n-type GaAscontact layer 606 as in Working Example 3 to form a GaInP epitaxialstacking structure. The sheet carrier density (n_(s)) at roomtemperature (300K) measured by means of the ordinary Hall effectmeasurement method was 1.6×10¹² cm⁻², and the average electron mobility(μ_(RT)) was 6400 (cm²/V·s). In addition, the n_(s) at the temperatureof liquid nitrogen (77K) was 1.5×10¹² cm⁻², and μ at 77K was 24,500cm²/V·s. Therefore, the GaInP epitaxial stacking structure provided withthe boron-doped spacer layer 604 according to this working exampleexhibited an electron mobility higher than that in the case of WorkingExample 3. In addition, virtually no hysteresis (looping) was found inthe drain current of a GaInP TEGFET constituted using the sametechniques as those recited in Working Example 3. In addition, theroom-temperature transconductance (g_(m)) measured at a source/drainvoltage of 3.0 V was high and homogeneous at 168 millisiemens (mS)/mm.

WORKING EXAMPLE 6

[0143] In this working example, the epitaxial stacking structure 9Ashown in FIG. 7 is formed upon an undoped semi-insulating (100) 2° offGaAs single-crystal substrate 901. The specific resistance of the GaAssingle crystal used as the substrate 901 is 2×10⁷ Ω·cm. Upon the surfaceof the substrate 901 with a diameter of 100 mm is deposited aconstituent part 902-1 of the first buffer layer constituting the bufferlayer 902 which has an Al_(L)Ga_(1−L)As/GaAs superlattice structure. Thesuperlattice structure 902-1 consists of an undoped Al_(0.30)Ga_(0.70)Aslayer 902 a with an aluminum composition ratio (=L) of 0.30 and anundoped p-type GaAs layer 902 b. The carrier density of theAl_(0.30)Ga_(0.70)As layer 902 a is 1×10¹⁴ cm⁻³ and its thickness is 45nm. The compensation ratio of the Al_(0.30)Ga_(0.70)As layer 902 a is1.0. The carrier density of the p-type GaAs layer 902 b is 7×10¹³ cm⁻³and its thickness is 50 nm. The compensation ratio of the p-type GaAslayer 902 b is 0.98. The number of stacking periods of the Al_(0.30)Ga_(0.70)As layer 902 a and p-type GaAs layer 902 b is 5 periods. TheAl_(0.30)Ga_(0.70)As layer 902 a and the p-type GaAs layer 902 b wereall formed at 640° C. by means of the low-pressure MOCVD method based ona (CH₃)₃Ga/(CH₃)₃Al/AsH₃/H₂ reaction system. The pressure at the time offilm formation was 1.3×10⁴ Pascal (Pa). Hydrogen was used as the carrier(transport) gas.

[0144] Upon the constituent part 902-1 of the first buffer layer 902 isstacked a GaAs layer 902 c deposited by means of a (C₂H₅)₃Ga/AsH₃/H₂reaction system low-pressure MOCVD method with the gallium sourcechanged from (CH₃)₃Ga to triethyl gallium ((C₂HI)₃Ga), forming a secondbuffer layer constituent part 902-2. The film formation temperature was640° C. and the pressure at the time of formation was 1.3×10⁴ Pa. Thecarrier density of the undoped n-type GaAs layer 902 is 2×10¹⁵ cm⁻³ andits thickness is 20 nm.

[0145] Upon the second buffer layer constituent part 902-2 is stacked anundoped n-type Ga_(0.80)In_(0.20)As layer deposited by means of alow-pressure MOCVD method using a (CH₃)₃Ga/(CH₃)₃In/AsH₃/H₂ reactionsystem as a channel layer 903. The carrier density of theGa_(0.80)In_(0.20)As layer constituting the channel layer 903 is 2×10¹⁵cm⁻³ and its thickness is 13 nm. The homogeneity of the indiumcomposition ratio was found to be 0.20±0.5% from the homogeneity of thephotoluminescence (PL) wavelength. The haze value of the surface of thislayer 903 measured from the intensity of scattering of incident laserlight was found to be 13 ppm.

[0146] Upon the Ga_(0.80)In_(0.20)As channel layer 903 is stacked aspacer layer 904 consisting of undoped n-type Ga_(0.51)In_(0.49)P bymeans of a low-pressure MOCVD method using a (CH₃)₃Ga/(CH₃)₃In/PH₃/H₂reaction system. The carrier density of the spacer layer 904 is 1×10¹⁵cm⁻³ and its thickness is 3 nm. The roughness of the surface of thespacer layer 904 was found to be 15 ppm as a haze value.

[0147] Upon the spacer layer 904 consisting of Ga_(0.51)In_(0.49)P isstacked an electron-supply layer 905 consisting of silicon-doped n-typeGa_(0.51)In_(0.49)P deposited by means of a low-pressure MOCVD methodusing a (CH₃)₃Ga/C₅H₅In/PH₃/H₂ reaction system. A hydrogen-disilane(Si₂H₆) gas mixture (concentration of 10 ppm by volume) was used as thesource of Si for doping. The carrier density of the electron-supplylayer 905 is 2×10¹⁸ cm⁻³ and its thickness is 30 nm. The homogeneity ofthe indium composition of the Ga_(0.51)In_(0.49)P constituting theelectron-supply layer 905 was found to be 0.49±0.5% from the homogeneityof the ordinary photoluminescence wavelength. The haze value measuredafter stacking this layer 905 was found to be 18 ppm.

[0148] Upon the surface of the electron-supply layer 905 consisting ofn-type Ga_(0.51)In_(0.49)P is stacked a contact layer 906 consisting ofSi-doped n-type GaAs by means of a (CH₃)₃Ga/AsH₃/H₂ reaction system. Theaforementioned hydrogen-disilane gas mixture was used as the source ofSi for doping. The carrier density of the contact layer 906 is 2×10¹⁸cm⁻³ and its thickness is 100 nm. The haze of the surface of the contactlayer 906 was measured to be 23 ppm. After the completion of epitaxialdeposition of the constituent layers 903-906 forming the epitaxialstacking structure 9A as such, it was heated to 500° C. in an arsine(AsH₃)-containing atmosphere, and thereafter cooled to room temperaturein a hydrogen atmosphere.

[0149] An ohmic electrode consisting of an indium-tin (In—Sn) alloy wasformed on the surface of the n-type GaAs contact layer 906 which formsthe uppermost surface layer of the epitaxial stacking structure 9A.Next, the ordinary Hall effect measurement method was used to measurethe electron mobility for the two-dimensional electron gas which travelsthrough the two-dimensional electron gas channel layer 903. The sheetcarrier density (n_(s)) at room temperature (300 Kelvin (K)) was1.6×10¹² cm⁻², and the average electron mobility (μ_(RT)) was 5500±9.%(cm²/V·s). In addition, the n_(s) at the temperature of liquid nitrogen(77K) was 1.4×10¹² cm⁻², and μ was 21,500 cm²/V·s, so a high electronmobility was exhibited.

[0150] After cooling, a patterning method employing knownphotolithography technology was used to create a recess in the surfaceof the n-type GaAs contact layer 906 which forms the surface-most layerof the epitaxial stacking structure 9A. Upon the n-type GaAs contactlayer 906 which remained as a mesa were formed a source electrode 907and a drain electrode 908. The source and drain ohmic electrodes 907 and908 were given a multi-layer structure consisting of gold-germanium (93%Au, 7% Ge by weight), nickel (Ni) and gold (Au) layers. The distancebetween the source electrode 907 and the drain electrode 908 was 10 μm.

[0151] Upon the surface of the Ga_(0.51)In_(0.49)P electron-supply layer905 exposed in the recess was formed a Schottky junction type gateelectrode 909 with a multi-layer structure consisting of a titanium (Ti)lower layer and an aluminum (Al) upper layer. The so-called gate lengthof the gate electrode 909 was 1 μm and the gate width was 150 μm.

[0152] The DC characteristics of the GaInP TEGFET, 9A were evaluated.The saturated source-drain current (I_(dss)) when a source/drain voltageof 3 Volts (V) was applied was found to be 70 milliampere (mA). When thedrain voltage was swept from 0 V to 5 V, virtually no looping(hysteresis) in the drain current was observed. The room-temperaturetransconductance (g_(m)) measured at a source/drain voltage of 3.0 V washigh and homogeneous at 155±5 millisiemens (mS)/mm. In addition, theleakage current flowing between the Au—Ge ohmic electrodes formed at aninterval of 100 μm exposed upon the surface of the buffer layer 902 wasfound to be less than 1 μA at 40 V, thus exhibiting high breakdownresistance. For this reason, the drain current pinch-off voltage became2.42±0.03 V, so a GaInP TEGFET with a homogeneous threshold voltage wasobtained.

WORKING EXAMPLE 7

[0153]FIG. 9 is a schematic cross section of the TEGFET, 123A accordingto this working example.

[0154] The epitaxial stacking structure 123A for TEGFET application isformed with an undoped semi-insulating (100) 2° off GaAs single crystalas its substrate 121. The specific resistance of the GaAs single crystalused as the substrate 121 is 3×10⁷ Ω·cm. Upon the surface of thesubstrate 121 with a diameter of 100 mm is deposited a buffer layer 122which has an Al_(L)Ga_(1−L)As/GaAs superlattice structure. Thesuperlattice structure consists of an undoped Al_(0.30)Ga_(0.70)As layer122 a with an aluminum composition ratio (=L) of 0.30 and an undopedp-type GaAs layer 122 b. The carrier density of the Al_(0.30)Ga_(0.70)Aslayer 122 a is 1×10¹⁴ cm⁻³ and its thickness is 45 nm. The carrierdensity of the p-type GaAs layer 122 b is 7×10¹³ cm⁻³ and its thicknessis 50 nm. The number of stacking periods of the Al_(0.30)Ga_(0.70)Aslayer 122 a and p-type GaAs layer 122 b is 5 periods. TheAl_(0.30)Ga_(0.70)As layer 122 a and the p-type GaAs layer 122 b wereall formed at 640° C. by means of the low-pressure MOCVD method based ona (CH₃)₃Ga/(CH₃)₃Al/AsH₃/H₂ reaction system. The pressure at the time offilm formation was 1×10⁴ Pascal (Pa). Hydrogen was used as the carrier(transport) gas.

[0155] Upon the buffer layer 122 is stacked a GaAs layer 123 depositedby means of a (C₂H,)₃Ga/AsH₃/H₂ reaction system low-pressure MOCVDmethod using triethyl gallium ((C₂H₅)₃Ga) as the gallium source. Thefilm formation temperature was 640° C. and the pressure at the time offormation was 1×10⁴ Pa. The carrier density of the undoped n-type GaAslayer 123 is 2×10¹⁵ cm⁻³ and its thickness is 20 nm.

[0156] Upon the GaAs layer 123 is stacked an undoped n-typeGa_(0.80)In_(0.20)As layer as the channel layer 124 deposited by meansof a low-pressure MOCVD method using a (CH₃)₃Ga/C₂H₅In/AsH₃/H₂ reactionsystem. The carrier density of the Ga_(0.80)In_(0.20)As layerconstituting the channel layer 124 is 1×10¹⁵ cm⁻³ and its thickness is13 nm. The homogeneity of the indium composition was found to be0.20±0.4% from the homogeneity of the ordinary photoluminescence (PL)wavelength. The haze value of the surface of this layer 124 measuredfrom the intensity of scattering of incident laser light was found to be12 ppm.

[0157] Upon the Ga_(0.80)In_(0.20)As channel layer 124 is stacked aspacer layer 125 consisting of undoped n-type Ga_(0.51)In_(0.49)P bymeans of a low-pressure MOCVD method using a (CH₃)₃Ga/C₅H₅In/PH₃/H₂reaction system. The carrier density of the spacer layer 125 is 1×10¹⁵cm⁻³ and its thickness is 3 nm. The haze value of the surface of thespacer layer 125 was measured to be 13 ppm.

[0158] Upon the spacer layer 125 consisting of Ga_(0.51)In_(0.49)P isstacked an electron-supply layer 126 consisting of Si-doped n-typeGa_(0.51)In_(0.49)P deposited by means of a low-pressure MOCVD methodusing a (CH₃)₃Ga/C₅H₅In/PH₃/H₂ reaction system. A hydrogen-disilane(Si₂H₆) gas mixture (concentration of 10 ppm by volume) was used as thesource of Si for doping. The carrier density of the electron-supplylayer 126 is 2×10¹⁸ cm⁻³ and its thickness is 25 nm. The homogeneity ofthe indium composition of the Ga_(0.51)In_(0.49)P constituting theelectron-supply layer 126 was found to be 0.49±0.5% from the homogeneityof the ordinary photoluminescence (PL) wavelength. The haze valuemeasured after stacking this layer 126 was found to be 18 ppm.

[0159] Upon the surface of the electron-supply layer 126 consisting ofn-type Ga_(0.51)In_(0.49)P is stacked a contact layer 127 consisting ofSi-doped n-type GaAs by means of a (CH₃)₃Ga/AsH₃/H₂ reaction system. Theaforementioned hydrogen-disilane gas mixture was used as the source ofSi for doping. The carrier density of the contact layer 127 is 2×10¹⁸cm⁻³ and its thickness is 100 nm. The haze of the surface of the contactlayer 127 was measured to be 23 ppm. After the completion of epitaxialdeposition of the constituent layers 122-127 forming the epitaxialstacking structure 123A as such, it was heated to 500° C. in an arsine(AsH₃)-containing atmosphere, and thereafter cooled to room temperaturein a hydrogen atmosphere.

[0160] An ohmic electrode consisting of an indium-tin (In—Sn) alloy wasformed on the surface of the n-type GaAs contact layer 127 which formsthe uppermost surface layer of the epitaxial stacking structure 123A.Next, the ordinary Hall effect measurement method was used to measurethe electron mobility for the two-dimensional electron gas which travelsthrough the two-dimensional electron gas channel layer 124. The sheetcarrier density (n_(s)) at room temperature (300 Kelvin (K)) was1.6×10¹² cm⁻², and the average electron mobility (μ_(RT)) was 5800±2%(cm²/V·s). In addition, the n_(s) at the temperature of liquid nitrogen(77K) was 1.5×10¹² cm⁻², and μ was 22,000 cm²/V·s, so a high electronmobility was exhibited.

[0161] After cooling, a patterning method employing knownphotolithography technique was used to create a recess in the surface ofthe n-type GaAs contact layer 127 which forms the uppermost surfacelayer of the epitaxial stacking structure 123A. Upon the n-type GaAscontact layer 127 which remained as a mesa were formed a sourceelectrode 128 and a drain electrode 129. The source and drain ohmicelectrodes 128 and 129 were given a multi-layer structure consisting ofgold-germanium (93% Au, 7% Ge by weight), nickel (Ni) and gold (Au)layers. The distance between the source electrode 128 and the drainelectrode 129 was 10 μm.

[0162] Upon the surface of the Ga_(0.51)In_(0.49)P electron-supply layer126 exposed in the recess was formed a Schottky junction type gateelectrode 120 with a multi-layer structure consisting of a titanium (Ti)lower layer and an aluminum (Al) upper layer. The so-called gate lengthof the gate electrode 120 was 1 μm.

[0163] The DC characteristics of the GaInP TEGFET, 123A were evaluated.The saturated source-drain current (I_(dss)) when a source/drain voltageof 3 Volts (V) was applied was found to be 70 milliampere (mA). When thedrain voltage was swept from 0 V to 5 V, virtually no looping(hysteresis) in the drain current was observed. The room-temperaturetransconductance (g_(m)) measured at a source/drain voltage of 3.0 V washigh and homogeneous at 160±5 millisiemens (mS)/mm. In addition, theleakage current flowing between the Au—Ge ohmic electrodes formed at aninterval of 100 μm exposed upon the surface of the buffer layer 122 wasfound to be less than 1 μA at 40 V, thus exhibiting high breakdownresistance. For this reason, the drain current pinch-off voltage became2.38±0.03 V, so a GaInP TEGFET with a homogeneous threshold voltage wasobtained.

[0164] As is evident from the aforementioned explanation, by means ofthe invention recited in claim 1, the electron-supply layer required toconstitute a GaInP TEGFET manifesting high transconductance isconstituted as a Ga_(X)In_(1−X)P layer with a gradient in thecomposition such that the gallium composition ratio decreases in thedirection of increasing layer thickness from the channel layer towardthe contact layer, so a two-dimensional electron gas efficientlyaccumulates in the interior of the channel layer, and a high electronmobility is manifested, so a GaInP epitaxial stacking structure with asuperior homogeneity in the transconductance and pinch-off voltage canbe provided.

[0165] By means of the invention recited in claim 2, the n-typeGa_(Y)In_(1−Y)P layer gradient-composition layer is constituted suchthat the gallium composition ratio is 1.0 at the junction interface withthe electron-supply layer and decreases to roughly 0.51 at the junctioninterface with the n-type GaAs contact layer, so a GaInP epitaxialstacking structure with a superior homogeneity in the transconductanceand pinch-off voltage can be provided.

[0166] By means of the invention recited in claim 3, an electron-supplylayer with superior lattice-matching characteristics with the GaAssingle-crystal substrate can be formed within the GaInP epitaxialstacking structure.

[0167] By means of the invention recited in claim 4, the n-typeGa_(Y)In_(1−Y)P layer gradient-composition layer is constituted suchthat the gallium composition ratio is 0.70 or greater at the junctioninterface with the electron-supply layer and decreases to roughly 0.51at the junction interface with the n-type GaAs contact layer, so a GaInPepitaxial stacking structure with a superior homogeneity in thetransconductance and pinch-off voltage can be provided.

[0168] By means of the invention recited in claim 5, by forming a regionof the electron-supply layer of a constant thickness from the junctioninterface with the channel layer as Ga_(Y)In_(1−Y)P with a constantgallium composition ratio (=Y), a stable junction barrier from theelectron-supply layer is given. In addition, a Ga_(Y)In_(1−Y)Pelectron-supply layer with superior homogeneity in the indiumcomposition ratio (=1−Y) and superior surface characteristics is given.

[0169] As is evident from the aforementioned explanation, by means ofthe invention recited in claim 6, the spacer layer required toconstitute a GaInP TEGFET manifesting high transconductance isconstituted as a Ga_(X)In_(1−X)P layer with a gradient in thecomposition such that the gallium composition ratio decreases in thedirection of increasing layer thickness from the channel layer towardthe contact layer, so a two-dimensional electron gas efficientlyaccumulates in the interior of the channel layer, and a high electronmobility is manifested, so a GaInP epitaxial stacking structure with asuperior homogeneity in the transconductance and pinch-off voltage canbe provided.

[0170] By means of the invention recited in claim 7, an electron-supplylayer with superior lattice-matching characteristics with the GaAssingle-crystal substrate can be formed within the GaInP epitaxialstacking structure.

[0171] By means of the invention recited in claim 8, the spacer layer isconstituted as an n-type Ga_(X)In_(1−X)P layer gradient-compositionlayer such that the gallium composition ratio is 0.70 or greater at thejunction interface with the electron-supply layer and decreases towardthe junction interface with the Ga_(0.51)In_(0.49)P electron-supplylayer, so a GaInP epitaxial stacking structure with a particularly hightransconductance can be provided.

[0172] By means of the invention recited in claim 9, the spacer layer isconstituted as an n-type Ga_(X)In_(1−X)P layer gradient-compositionlayer such that the gallium composition ratio is 1.0 at the junctioninterface with the electron-supply layer and decreases toward thejunction interface with the Ga_(0.51)In_(0.49)P electron-supply layer,so a GaInP epitaxial stacking structure with a particularly high andhomogenous transconductance can be provided.

[0173] By means of the invention recited in claim 10, the spacer layeris constituted as an n-type Ga_(X)In_(1−X)P layer gradient-compositionlayer such that the gallium composition ratio decreases to 0.51±0.01 atthe junction interface with the electron-supply layer, so a GaInPepitaxial stacking structure with a particularly high and homogenoustransconductance can be provided.

[0174] By means of the invention recited in claim 11, the spacer layeris constituted as a boron-doped, low-carrier density, high-resistanceGa_(X)In_(1−X)P gradient-composition layer, so a high-electron-mobilitytwo-dimensional electron gas efficiently accumulates in the interior ofthe channel layer, so a GaInP epitaxial stacking structure with superiortransconductance characteristics can be provided.

[0175] By means of the invention recited in claim 12, the buffer layeris constituted as a portion with a superlattice periodic structureconsisting of Al_(L)Ga_(1−L)As (0≦L≦1) layers with different aluminumcomposition ratios (L) vapor-deposited using an organic methyl compoundof aluminum or gallium as its starting material, and a portion havingAl_(M)Ga_(1−M)As (0≦M≦1) vapor-deposited using an organic ethyl compoundof Al or Ga as its starting material touching, so a high-resistancebuffer can be constituted, and a method of fabricating a GaInP epitaxialstacking structure with a low leakage current and the epitaxial stackingstructure can be provided.

[0176] In particular, by means of the invention recited in claim 13, thesuperlattice periodic structure constituting one part of the bufferlayer with a periodic alternating layer structure of Al_(L)Ga_(1−L)Aslayers vapor-deposited using an organic methyl compound as its startingmaterial and with a stipulated compensation ratio, so a GaInP epitaxialstacking structure with a low leakage current can be provided.

[0177] Furthermore, in particular, by means of the invention recited inclaim 14, the superlattice periodic structure constituting one part ofthe buffer layer with a periodic alternating layer structure ofAl_(L)Ga_(1−L)As layers and p-type GaAs layers vapor-deposited using anorganic methyl compound as its starting material and with a stipulatedcompensation ratio and carrier density, so a GaInP epitaxial stackingstructure with a particularly low leakage current can be provided.

[0178] By means of the invention recited in claim 15, anAl_(M)Ga_(1−M)As layer vapor-deposited using an organic ethyl compoundas its starting material is provided joined to a Ga_(Z)In_(1−Z)Aschannel layer, so the channel layer can be formed from aGa_(Z)In_(1−Z)As layer which has a homogenous indium composition andalso low deterioration in the surface state arising from segregation ofindium or the like, so a GaInP epitaxial stacking structure withsuperior homogeneity in the electron mobility and transconductance, andsuperior homogeneity in the pinch-off voltage can be provided.

[0179] By means of the invention recited in claim 16, anAl_(M)Ga_(1−M)As layer vapor-deposited using an organic ethyl compoundas its starting material is formed from n-type Al_(M)Ga_(1−M)As with astipulated carrier density and thickness, so the channel layer andelectron-supply layer can be formed from an indium-containing GroupIII-V compound semiconductor with a superior homogeneity in its indiumcomposition, so a GaInP epitaxial stacking structure with a homogeneouspinch-off voltage and g_(m) can be provided.

[0180] By means of the invention recited in claim 17, the thickness ofthe n-type Al_(M)Ga_(1−M)As layer vapor-deposited using an organic ethylcompound as its starting material is set to be no greater than thethickness of the Al_(L)Ga_(1−L)As layer vapor-deposited using an organicethyl compound as its starting material constituting the superlatticeperiodic structure, so a GaInP epitaxial stacking structure withparticularly low hysteresis in the drain current can be provided.

[0181] By means of the invention recited in claim 18, the aluminumcomposition ratio (M) of the n-type Al_(M)Ga_(1−M)As layervapor-deposited using an organic ethyl compound as its starting materialis set to be no greater than the aluminum composition ratio (L) of anyof the Al_(L)Ga_(1−L)As layers which constitute the superlatticeperiodic structure, so a GaInP epitaxial stacking structure withparticularly reduced hysteresis in the drain current can be provided.

[0182] By means of the invention recited in claim 19, a GaAs thin-filmlayer vapor-deposited using triethyl gallium as its starting material isused as the substrate layer when an indium-containing Group III-Vcompound semiconductor layer is provided, so a Ga_(Z)In_(1−Z)As layerwhich has a superior homogeneity in its indium composition, and asuperior surface roughness value, along with a Ga_(X)In_(1−X)P spacerlayer and electron-supply layer can be formed, and therefore, a methodof fabricating a GaInP epitaxial stacking structure with a superiorhomogeneity in the transconductance and pinch-off voltage and theepitaxial stacking structure can be provided.

[0183] By means of the invention recited in claim 20, the channel layeris formed from n-type Ga_(Z)In_(1−Z)As with a stipulated surfaceroughness, so therefore a GaInP epitaxial stacking structure withsuperior homogeneity in the transconductance and pinch-off voltage canbe provided.

[0184] By means of the invention recited in claim 21, the spacer layeris formed from n-type Ga_(X)In_(1−X)P with a stipulated surfaceroughness, so therefore a GaInP epitaxial stacking structure withsuperior homogeneity in the transconductance and pinch-off voltage canbe provided.

[0185] By means of the invention recited in claim 22, theelectron-supply layer is formed from Ga_(Y)In_(1−Y)P doped with n-typeimpurities, having a stipulated surface roughness, so therefore a GaInPepitaxial stacking structure with superior homogeneity in thetransconductance and pinch-off voltage can be provided.

[0186] By means of the inventions recited in claims 23 and 24, then-type Ga_(Z)In_(1−Z)As channel layer, Ga_(X)In_(1−X)P spacer layer andelectron-supply layer are formed by means of a chemical vapor depositionmethod using cyclopentadienyl indium as the starting material forindium, so a channel layer, spacer layer and electron-supply layer withsuperior homogeneity of the indium composition and little surfaceroughness are formed, and moreover, a method of fabricating a GaInPepitaxial stacking structure and the epitaxial stacking structure with asuperior homogeneity in the transconductance and pinch-off voltage canbe provided.

[0187] By means of the invention recited in claim 26, a field effecttransistor with a particularly high electron mobility can be provided.

What is claimed is:
 1. A GaInP epitaxial stacking structure comprising:stacked upon a GaAs single-crystal substrate, at least a buffer layer, aGa_(Z)In_(1−Z)As (0<Z≦1) channel layer, and a Ga_(Y)In_(1−Y)P (0<Y≦1)electron-supply layer joined to said channel layer, the GaInP epitaxialstacking structure being characterized in that it includes a regionwithin the electron-supply layer wherein the gallium composition ratio(Y) decreases from the side of the junction interface with the channellayer toward the opposite side.
 2. A GaInP epitaxial stacking structureaccording to claim 1, characterized in that the gallium compositionratio of the electron-supply layer is Y≧0.51±0.01.
 3. A GaInP epitaxialstacking structure according to claim 1 or claim 2, characterized inthat the gallium composition ratio of the electron-supply layer at thejunction interface with the channel layer is Y≧0.70.
 4. A GaInPepitaxial stacking structure according to claim 1 or claim 2,characterized in that the gallium composition ratio of theelectron-supply layer at the junction interface with the channel layeris Y=1.0.
 5. A GaInP epitaxial stacking structure according to any ofclaims 1-4, characterized in that at the junction interface between theelectron-supply layer and the channel layer, there is a region with athickness in the range 1-20 nanometers wherein the gallium compositionratio is constant.
 6. A GaInP epitaxial stacking structure comprising:stacked upon a GaAs single-crystal substrate, at least a buffer layer, aGa_(Z)In_(1−Z)As (0<Z≦1) channel layer, a Ga_(X)In_(1−X)P (0<X≦1) spacerlayer, and a Ga_(Y)In_(1−Y)P (0<Y≦1) electron-supply layer, the GaInPepitaxial stacking structure being characterized in that the channellayer, spacer layer, and electron-supply layer touch each other in thisorder, and that the GaInP epitaxial stacking structure includes a regionwithin the spacer layer wherein the gallium composition ratio (X)decreases from the side of the junction interface with the channel layertoward the side of the electron-supply layer.
 7. A GaInP epitaxialstacking structure according to claim 6, characterized in that thegallium composition ratio of the electron-supply layer is Y=0.51±0.01.8. A GaInP epitaxial stacking structure according to claim 6 or claim 7,characterized in that the gallium composition ratio of the spacer layerat the junction interface with the channel layer is X≧0.70.
 9. A GaInPepitaxial stacking structure according to claim 6 or claim 7,characterized in that the gallium composition ratio of the spacer layerat the junction interface with the channel layer is X=1.0.
 10. A GaInPepitaxial stacking structure according to any of claims 6-9,characterized in that the gallium composition ratio of the spacer layerat the junction interface with the channel layer is X=0.51±0.01.
 11. AGaInP epitaxial stacking structure according to any of claims 6-10,characterized in that a boron-doped n-type layer constitutes the spacerlayer.
 12. A GaInP epitaxial stacking structure according to any ofclaims 1-11, characterized in that the buffer layer consists of aperiodic structure of a plurality of Al_(L)Ga_(1−L)As (0≦L≦1) layerswith different aluminum composition ratios (L) vapor-deposited using anorganic methyl compound of aluminum or gallium as its starting material,and that the GaInP epitaxial stacking structure has an Al_(M)Ga_(1−M)As(0≦M≦1) layer vapor-deposited using an organic ethyl compound ofaluminum or gallium as its starting material touching said periodicstructure.
 13. A GaInP epitaxial stacking structure according to claim12, characterized in that the relationship 0.9≦K≦1.0 holds true for thecompensation ratios (K) (K=N_(a)/N_(d) (if N_(a)≦N_(d)) andK=N_(d)/N_(a) (if N_(d)<N_(a)); N_(a): acceptor density of theconstituent layer, N_(d): donor density of the constituent layer) of theconstituent layers of the periodic structure.
 14. A GaInP epitaxialstacking structure according to claim 12 or claim 13, characterized inthat the periodic structure consists of an Al_(L)Ga_(1−L)As (0≦L≦1)layer and a p-type GaAs layer, and that the carrier density of eachconstituent layer is 1×10¹⁵ cm⁻³ or less.
 15. A GaInP epitaxial stackingstructure according to any of claims 12-14, characterized in that theAl_(M)Ga_(1−M)As layer is touching the channel layer.
 16. A GaInPepitaxial stacking structure according to any of claims 12-15,characterized in that the Al_(M)Ga_(1−M)As layer has a carrier densityof 5×10¹⁵ cm⁻³ or less and a thickness of 100 nm or less, and consistsof an n-type layer.
 17. A GaInP epitaxial stacking structure accordingto any of claims 12-16, characterized in that the thickness of theAl_(M)Ga_(1−M)As layer is less than the thickness of the constituentlayers of the periodic structure.
 18. A GaInP epitaxial stackingstructure according to any of claims 12-17, characterized in that thealuminum composition ratio (M) of the Al_(M)Ga_(1−M)As layer is lessthan the aluminum composition ratio (L) of the Al_(L)Ga_(1−L)As layerswhich constitute the periodic structure.
 19. A GaInP epitaxial stackingstructure according to any of claims 6-18, characterized in that thebuffer layer comprises an Al_(L)Ga_(1−L)As (0≦L≦1) layer vapor-depositedusing a trimethyl compound of a Group III element as its startingmaterial, that a GaAs layer vapor-deposited using triethyl gallium asthe starting material for gallium is disposed between the buffer layerand channel layer, that the channel layer has a conduction type ofn-type, that the spacer layer and electron-supply layer are n-typelayers vapor-deposited using trimethyl gallium as the starting materialfor gallium, that the homogeneity in the indium composition ratio withineach of the spacer layer and electron-supply layer is ±2% or less, andthat the spacer layer and electron-supply layer are touching each other.20. A GaInP epitaxial stacking structure according to any of claims6-19, characterized in that the surface roughness (haze) after formationof the channel layer is 60 ppm or less, and that the channel layertouches a GaAs layer vapor-deposited using triethyl gallium as thestarting material for gallium.
 21. A GaInP epitaxial stacking structureaccording to any of claims 6-20, characterized in that the spacer layerand channel layer touch each other, and that the surface roughness(haze) after formation of the spacer layer is 100 ppm or less.
 22. AGaInP epitaxial stacking structure according to any of claims 6-21,characterized in that the surface roughness (haze) after formation ofthe electron-supply layer is 200 ppm or less.
 23. A method offabricating a GaInP epitaxial stacking structure according to any ofclaims 1-5 or 12-22, comprising: a step wherein the buffer layer isvapor-deposited using an organic methyl compound of aluminum or galliumas its starting material, a step wherein an AlGaAs layer isvapor-deposited using an organic ethyl compound of aluminum or galliumas its starting material in contact with said periodic structure, and astep wherein the channel layer and electron-supply layer are formed bymeans of a chemical vapor deposition method using cyclopentadienylindium which has a bond valence of monovalent as the starting materialfor indium.
 24. A method of fabricating a GaInP epitaxial stackingstructure according to any of claims 6-22, comprising a step wherein thebuffer layer is vapor-deposited using an organic methyl compound ofaluminum or gallium as its starting material, a step wherein an AlGaAslayer is vapor-deposited using an organic ethyl compound of aluminum orgallium as its starting material in contact with said periodicstructure, and a step wherein the channel layer, spacer layer andelectron-supply layer are formed by means of a chemical vapor depositionmethod using cyclopentadienyl indium which has a bond valence ofmonovalent as the starting material for indium.
 25. A GaInP epitaxialstacking structure fabricated using the method of fabricating a GaInPepitaxial stacking structure according to claim 23 or claim
 24. 26 Afield effect transistor fabricated using the GaInP epitaxial stackingstructure according to any of claims 1-22 and 25.